natalie,

You can combine miles from up to 3 accounts for a single ticket.... so 12,000 from each of 3 accounts can be used for a single 35,000 ticket.

You can earn TA miles from flights on AA... just give your TA number.

I think miles are good for 3 years- but you can verify the expiration date on the taca website if you log into your account. The expiration date is a drop-dead date: unlike other programs where you just need to have some kind of activity every 3 years, the TACA miles truly disappear after 3 years if they're not used.

Hope this helps some.