You can set the fastest speed that the phone will pick up in cellular in settings. However if you have the fastest speed set to 3g but you receiving a 2g signal then you cant change, it will happen automatically.
Just change the hgihest speed back to 2g you might notice the cellular network gone when it comes back set the highest speed back to 3g again it'll go and you will shift to 3g thereafter. It worked for me cause I tried and didn't have to wait till it automatically shifted. That's a quick way to shift I guess