Hi, so being too lazy or something to have 412 with 215 to cover my stoner/sludge/alt needs, I`ve decided to build one cab with 2 12" and 1 15" speakers.

15" will be Celestion Fullback which is 8 ohm (and 100W)
12" will be two G12K 100 (because they are 100W) they are 8 ohm or 16 ohm

Now, my question is - how should I wire them? The placement in cab is 12s next to each other on top, 15 on the bottom.

I need this cab to be 8 ohm (or 16 ohm) because my current amp can`t be set for 4 ohm. So am I right if I assume it should be two 16 ohm 12s together in paralel which would give me 8 ohm and this connect to remaining 8 ohm 15 is series to get 16 ohm? Or is there other and better way?
Last edited by blackhaulmike at May 10, 2017,
Wire the two 12" speakers (16 ohm) in parallel to give you 8 ohms from that string. 
The 15" speaker is already 8 ohms. 

Now wire that whole mess (the two sections) in serial, which will give you 16 ohms. Just make sure that you don't wire them out of phase, or it will sound very odd <G>

Here's a suggestion, however. Look for something from ISP Technologies called the Vector before you start building. You may not be able to find one new, but it's SO worth finding one, period. It's a 30" x 30" (412-size) box with two 12" speakers and one 15" speaker. But here's the major difference. The 15" speaker is a subwoofer, it's in its own sealed and ported compartment inside the 4x12-sized box, it's powered by it's own 400W solid state amp, and there are some electronics that direct the mids/highs to the 12" speakers and the lows to the 15" speaker. However they do it, the full power of the head is directed to the 12" speakers as well, which makes them louder and more efficient and easily able to handle the power. 

The 15" speaker, with the 400W power amp, is free to handle low end and it has enough power to do that, and the response of that internal subwoofer system is amazing for stoner/sludge/alt. It will curl your narglies. 
Last edited by dspellman at May 10, 2017,
dspellman is right about the connections and I like the idea to separate zones inside the cab - I would suggest this is a good idea even if they are connected to the same amp.