well, AOE3 is fairly modern. The GMA950 in the macbook is not very powerful, and AOE3 can use quite a lot of graphics card power at higher settings.
You should be able to play it at the macbook's native resolution with most of the effects set to "medium" or lower and have a pretty playable experience, at least if you have more than the original amount of RAM. as with all things on OS X, the more RAM the better.
Most modern RTS games use a lot of graphics power and a lot of processor power, too. For example Company of Heroes runs pretty slowly on my PC which has a 6600 GT.
To put it mildly, the GMA950 is not as good as the 6600 GT.
But if a game is from the AOE3 era or OLDER, the MB will play it just fine if you're willing to make some graphical concessions.