Looking at it objectively Lowell is in the top 3 in 3B this year but its hard to tell whos the "best" so far. Glaus and AROD are both there with him if you consider just offense. Here is how Lowell ranks by AL 3B this year (Rank is Lowell's Rank respective to other AL 3rd basemen) :
Category - Rank - Leader
OPS - 2nd - Glaus
SLG - 2nd - Glaus
OBP - 3rd - AROD, Blalock
AVG - 1st
Total Bases - 2nd - Glaus
2B - 1st
HR - 8th - Glaus, AROD
RBI - 7th - Glaus, AROD
R - 6th - AROD, Glaus
The only categories that Lowell leads among AL 3B is batting average and doubles. While good he still is still behind Glaus in OPS, SLG, Total Bases, HR, RBI, R and behind AROD in OBP, HR, RBI, and R. Obviously defense is important but thats not how All-Star teams are usually constructed. Like I said before Lowell is top 3 but I wouldn't say he's number 1.