3:1 So [Therefore] when we could stand it [to bear up under the strain of separation] no longer [communication was slow; Paul anxious to hear about the welfare of the Thessalonian believers], we thought it best [to determine] to be left by ourselves [alone; “He shows that he had a greater consideration for them than for himself by choosing to be left alone rather than that they should be deserted.” (Calvin)] in Athens [cf. Acts 17:16-34].
3:2 We [refers either to Paul or to Paul and Silas] sent [a tangible expression of Paul’s concern for the Thessalonian believers] Timothy [Paul’s “true son in the faith” (1 Tim. 1:2)], who is our brother [points to the spiritual companionship that existed between Paul and Timothy] and God’s fellow worker [co-worker for God; Gr. word “deacon” (used in broad sense of one who ministers in God’s service)] in spreading the gospel of Christ [Christ is author and content of the gospel], [threefold reason for sending Timothy]  to strengthen [support; establish (because their faith under attack)] and  encourage [Gr. parakaleo : to exhort, to comfort, to inspire] you in [for the furtherance of; on behalf of] your faith,
3:3  so [the result of Timothy’s ministry] that no one would be unsettled [to shake or wag (like a dog’s tail)] by these trials [pressure; tribulations]. You know quite well that we [Paul, who had suffered numerous trials (cf. 2 Cor. 11:24-28), included himself with the Thessalonians] were destined [Jesus had predicted the suffering of His followers (Matt. 10:17-42; 24:9; Jn. 15:20)] for them [trials].
3:4 In fact, when we were with you, we kept telling [told on several occasions; Paul did not hide the cost of following Christ (cf. Acts 14:22)] you [therefore persecution should not have surprised the Thessalonians (cf. 1 Pet. 4:12)] that we would be persecuted [to put under pressure; cf. Jn. 15:20]. And it turned out [to happen] that way, as you well know.
3:5 For this reason, when I could stand it no longer [the suspense concerning the spiritual welfare of the Thessalonian believers], I sent [another purpose of Timothy’s visit] to find out about your faith [to discover if their faith was holding up to persecution]. I was afraid that in some way the tempter [Satan determined to spiritually defeat believers] might have tempted [to test, to try, to tempt; indicates Satan already at work] you and our efforts [toil; Paul labored to point of weariness] might have been useless [cf. Phil. 2:16].
3:6 But Timothy has just now [indicates that Paul wasted no time in writing to the Thessalonians after Timothy’s arrival in Corinth] come [after completing his assigned mission (3:2); cf. Acts 18:1-5] to us from you and has brought good news [Timothy’s report was a “gospel” to Paul’s anxious heart; the only time Paul used the term translated “good news” (Gr. euangelizomai) for anything other than preaching the gospel; Timothy’s report confirmed that Paul’s work in Thessalonica was not wasted (cf. 3:5)] about your faith [Paul’s principal concern (cf. 1 Thess. 3:5,7); their faith remained strong in the midst of persecution] and love [demonstrated toward one another]. He has told us that you always have pleasant memories [indicates that the Thessalonians did not view Paul as a troublemaker or charlatan preacher] of us [the founder of their church] and that you long to see us, just as we also long to see you.
3:7 Therefore, brothers, in all our distress [choking pressures] and [cf. Acts 18:5-6,12-13 re: what Paul was experiencing from Jewish community and local government in Corinth] persecution [overbearing tribulation] we were encouraged [had new energy to go on; cf. Rom. 1:12] about you because of your faith.
3:8 For now we really live [Paul’s life had “been on hold” until he received Timothy’s report], since [the basis for Paul’s encouragement] you are standing firm [faithfulness despite difficult circumstances] in the Lord [supplies the power to stand firm].
3:9 [a rhetorical question] How can we thank God enough for you in return for all the joy we have in the presence of our God because of you?
3:10 Night and day [constantly (as in 2:9)] we pray most earnestly [cf. 2 Cor. 11:28 re: Paul’s “concern for all the churches”] that we may see you again [a return visit] and supply [“to put into the proper condition, to make complete;” reinforce, improve the quality of a person’s relationship with God] what is lacking [Paul had only been with them a short time, thus they needed further teaching that would lead them to maturity in Christ; cf. Eph. 4:12-13] in your faith [refers to Christian maturity not salvation experience].
3:11 Now may our God and Father himself [our lives are in His hands] and our Lord Jesus clear the way [to make a straight path (from Paul to the Thessalonians); to remove the obstacles which Satan had used to obstruct the path (cf. 2:18)] for us to come to you [no indication that Paul was able to return to Thessalonica; note that Aristarchus and Secundus (from Thessalonica) joined Paul on his third missionary journey (Acts 20:4-5)].
3:12 May the Lord make your love [agape kind of love: selfless desire for the welfare of others; essential for the survival and growth of a church] increase and overflow [love should be growing continually; cf. Rom. 13:8] for each other [cf. Jn. 13:34] and [additionally] for everyone else [those outside the church], just as ours does for you.
3:13 May he strengthen your hearts [inward purposes and desires] so that you will be blameless [spotless; without fault] and holy in the presence of our God and Father when [refers to second coming of Christ] our Lord Jesus comes with all [includes angels as per Matt. 13:41; 25:31; Mk. 8:38; 13:27; Lk. 9:26; 2 Thess. 1:7] his holy ones [believers as per 4:13-18; coming with departed saints for the living saints].