As the 30m band is CW-only, I will skip this band for the moment. I'm not (yet) a CW guy.
Leaving 17m an 12m to combine in one antenna.
How to make the antenna resonant for both bands. The frequencies relate to each other as 3 to 4.
On the Internet there's lot of discussion about this End Fed.
One thread uses a full wave for 12m a coil and after that a wire of less than a meter to make 17m and 30m resonant. But how dus this work in theory?
So I dicide to start with one band: 17m.