Nah, avengers 3 is going to be cosmic based on fiege's words. Whether or not it's Thanos, I don't know. HOWEVER, fiege said events that lead to the culmination of phase 2 will also lead up to the culmination of phase 3. Meaning things that contribute to the end of phase 2, will be a factor throughout phase 3. Since phase 2 is cosmic (with Thanos and gotg), and since the end of phase 2 (whether it be the defeat of thanos or not) will most likely be cosmic reason, it is safe to say, in my opinion that because of Fiege's words on things playing over, that we probably won't ultron or the master's until atleast phase 4.
However, they could POSSIBLY do a MOS movie with Thanos, somehow, Thanos can still be the villain and use the team for whatever reason, and then have Thanos in phase 3. Atleast having the MOS as villains for avengers 2 will sorta keep the whole cosmic thing down a little bit. Having Thanos completely done in phase 2, and then said established threat carrying into phase 3 (cosmic threat) it's too much cosmic related villains.
However, with possibly Crossbones in cap, Abomination still being around, The Manderin, Radioactive Man, Malekith (Kurse if he's alive) These are 3 and 4 characters alone introduced that could sort of be made into the masters of evil, plus what villains we still have. There seems to be a lot of villains being created, so they very well could be trying to work a masters of evil story line into Thanos' story.
Interesting to see what will play out.
I just think having Thanos play out in 2, and then ANOTHER cosmic type threat in phase 3 is too much. If avengers 2 is MOS, it can atleast take a little bit away from the whole cosmic side of it. I mean, the IG isn't even introduced yet, I hope we get more than just 1 movie to feel the damn impact and scale of this weapon.