How to cut a rectangular notch out of a timber beam?
The third cut is the tricky one, you could plunge cut with an oscillating tool, that'd be one way.

However the old-school way is probably faster and easier just to make a few more cuts between the first two, and then break the fragments out and clean it up using a chisel.

if the first cuts were with a hand-held rotary saw, you already have the depth set, so make another three-to five cuts kind of parallel between the first two (precision is not needed) then snap off the upstands and scrape the bottom clean with the chisel.