Welcome to CC platinimhawk. As you say it is all about straights.
A8 is only better than A3 when we are up against at least one low card, such as 66 or A7, against two high cards we are slightly better with A3. It is worth considering some scenarios to see how this works.
Suppose we are all in and the villain shows TT. If the board pairs our kicker we are no better off - it doesn't matter if it is a 3 or an 8, if we hit trips with our kicker we are ahead but again it doesn't matter if it is a set of 3s or 8s. If we hit a flush with our kicker, again it makes no difference whether it is a 3 or an 8. When it comes to straights there are two things to consider: with A3 we only need three cards for our straight but with A8 we need four; also, because our villain is holding two tens there are two fewer cards that can be dealt that will help us. Even if one of the 'helpful' tens comes down and gives us a straight it gives the villain a full house draw.
Our A3 is about 29% to TT and A8 is about 28%, against KQ we are 57% and 56.5% respectively. This is not enough of a difference to say that A3 should be played and A8 folded. What it does mean is that if we are going to shove steal, resteal or make a desperate call A3 is usually as good as or better than A8 against the sort of hands that the villain would play.
I do not claim that either hand is one we really want to take seriously, but I am sure a lot of players play A8 a lot more than A3 because it looks so much stronger. In reality it only has a big advantage when the villain holds one or more small cards.