Given that the shuttle SRB has 3.3 million lbs thrust and you generally
need two stages to get to orbit, would it be feasible to use a fisrt
stage consisting of the two SRB and a 2nd stage consisting of 1 SRB
with payload on top. You would need a 3rd stage I assume that is
controllable.

What about clustering more than 2 SRB for stage 1 to make a huge Big
Dumb Booster?