So, the answer's a little complicated. The only allowance that you could really insist upon, if things don't work out, is the 0+1 domestic allowance. However, you're quite likely to get 2+1 bags, assuming TPE is your final destination, because that's UA's allowance for US-TPE flights. Assuming that the agent tags your bags through properly, they'll do that by creating a dummy segment on your XXX-SFO reservation, so that it looks like an XXX-SFO-TPE reservation -- at which point the TPE allowances will kick in when the agent goes to print the bag tags.

However, if anything goes sideways, or if you get an agent who decides he wants to try out a side gig in revenue enforcement or something, you wouldn't really have a leg to stand on.

If you need 3 bags, and don't want to run the risk of UA charging you an arm and a leg, I would watch the upgrade price for your XXX-SFO flight and buy up if it seems reasonable. Then, you'd definitely get the 3 bags. (If you're flying XXX-YYY-SFO, you can buy up either XXX-YYY or YYY-SFO). If you're the gambling sort, you could wait until check-in -- if they don't want any extra baggage fees, you're golden; if they do, ask about the cost of the buy-up and make a decision.

Hope this helps...