General practice, unless you're a jazz player, is to start and begin a solo on the root note of the scale. If you're playing in A major, you'd start your solo on A and eventually resolve back to A. To find the pentatonic scale for A, jump down a step and a half to the relative minor, F#. The F# minor, A major, pentatonic scale should sound just fine over that I-IV-V progression you mentioned.