Does arm strength only show up in +/- plays? The reason I ask is I'm curious if playing a 85/85/60/60 guy at 3rd would be okay. Would hate to miss out on a bunch of assists or whatever and not realize it. Thanks.

Also, I assume there is a penalty for playing a lefty at 3rd? or was that just assigned to 2nd?

LH 3B is a no-no. I wouldn't do it even if he had a decent AS/AA rating, which he doesn't.

1)  There is a penalty for playing a LH at 3B, but it's way too small; you can play a guy that throws left but has all 4 key defensive ratings 75-85 at 3B without it being the disaster it should be.
2)  But not this guy if he throws left; the penalty plus the weak arm will make it untenable.
3)  But you can play this guy at 3B if he throws right; you'll probably have more + than - plays, and the glove will make the fielding pct OK (understanding that throwing errors are worse than dropping errors because of the extra base advance).

I don't have a left hander, it was just a secondary question to a more important question, that being do +/- plays tell the whole story on arm strength?

I think they tell a good part of the story on arm strength.  Do they tell 100%?  Only ADMIN knows.