I would suggest SQ or UA. UA would status match up to 1P, but I have not heard about SQ doing any status matching.

A3 is an viable option, but A3 will credit much lower % miles with lower fare buckets compared to UA, which is 100% for all (and even for some partner airlines like LH and NH)

Perhaps the best option is to match with UA so you get temporary *G status, then credit flight miles on SQ for long term. It would also depend on whether you will move back to the US anytime soon, if so, then UA is better.