[oil pan 4]

Romex 12/3 with ground is black, red, white and the bare ground.
12/3 flexible cord is only 3 wires.

You can only put the number or circuits listed on the box in the box, so your range and dryer are 1 circuit each. Say you have a 20 circuit panel with 20 breaker slots or 30 circuits with 30 slots. Lets say its a 20/20 box and It's all filled up with say 3 double pole breakers and 14 single pole. So you got 17 circuits. You can only switch out 3 single pole breakers for 1/2 size.
I don't believe I have ever seen half size double pole breakers. You might be able to link 2 half size home line breakers together it would be sketchy. I say cram the standard 15 or 20 amp branch circuits together that supply power to bedrooms and lights to make room and put the 240v circuit on a full size 2 pole breaker.

What I do is buy over sized outdoor panels with usually double the number of circuits as there are spaces. I could fill that box with all 1/2 breakers and it woud be perfectly up to code. For my sub panels I have been using outdoor 125 amp 12 space 24 circuit panels.