Help with a new cp concept I have...

Thats Territory control.

You literally just described a gamemode thats been in the game since it launched.

hydro!

You didn't mention territorial control anywhere in your post
But here's something you might want to read
And here's a link to ABS resource pack that contains the gamemode already done for you

EDIT: tc_hydro isn't exactly how you described but it's the only tc map in the game so you can base your specific logic off of that.

This proposed gamemode sounds somewhat different to TC. It's more linear and each stage has different amounts of control points. Though I've not properly looked at it, I imagine the TC logic is a lot more complex than what this would require.

I'm afraid I don't have a solution, but it may be worth looking at the gamemode library found in the ABS pack soosh has linked.

I haven't tested this, but I have a solution that should work, theoretically:

Since map stages are handled by team_control_point_round entities, which are normally marked with Priority values so that they're played in a specific order, you may be able to force different rounds to play based on the outcome of the first one by sending a Disable input to the one you don't want to play.

So, in your case you have three t_c_p_r's: one for the 3cp round, and one each for the attack/defend rounds. The 3cp round is first, so it will have Priority value 1. The other two will both be 0. After the 3cp round is played and a winner decided, disable the t_c_p_r for the winning team's stage (perhaps as an output fired from the last control point upon capture), and the other stage should be selected since it is the only one remaining.

actually... In my opinion, TC could be a hybrid ctf/koth map. it is territory.... isn't it?