Assuming the Celts can trade for KLove (PF) so you have Big 2 in him and Rondo (PG) - who will be the final star in Big 3? It will have to be either a SG or SF or C. Here are the rankings of top players that are free agents or rumored as even remotely possible trade material. Most players probably do not fit Big 3 status so may have to wait to 2015.
SG - DeRozan, Stephenson, Afflalo, Hayward, Thompson
SF - Anthony, Gay, Parsons, Deng,
C - Noah, Cousins, Gasol, Gortat
My opinion is whether you like him or not that Anthony is only true Big 3 material and is a FA. Who is your third Big 3?
Dream Scenario/Unload All Assets/Win Now: PG-Rondo/SG-Afflalo or KMart or Bradley/SF-Anthony/PF-Love/C-Asik