or rather he would of guess Elrond, Gal, Cirdan.. Cir gave his to Gandalf... but Sauron wouldn't have known that Gandalf couldof had one.. but previously to the istari coming to M.E he would have been correct.. So it was the matter of getting them so to speak..

He prob could have taken out Lorien from Dol Guldur.. but he only would of had one.. he need to to conquest all of M.E at once to get all three.. otherwise if he attacked Lorien the others would of hurried there attack... to stop him from getting the other two...

Although he would have the problem of finding Gandalf.. wondering around all over the place... at least when Cir had the ring he knew were he was (for most of the time).

Beren Valthôl