I like tube fin rockets and build them almost exclusively.
Don't bother with sim programs because most of them "can't handle the truth!" Yes, you can approximate using three fins for each tube although some say you should use 19 fins. I've also heard there are other considerations beside just the fin area that come into play but 18 or 19 fins is probably close enough government work.
I usually figure that the CP of a tube fin rocket is down inside the fin can...maybe 2/3 up from the bottom. My tube fins are usually shorter than the tube fin diameter by about 10-15 percent. By that I mean a 4" diameter tubefin is only 3-1/2" tall. Aspect ratio, if you will. I have two rockets where I have built the tube fins even shorter than that. These are the two rockets in the foreground of the above photo #1. If you examine the length of the tube fins in the above photo #2, you can see the fins of the foreground rocket are shorter than the rocket behind it. Both tube fins are the same diameter.
As to the number tube fins, six tubes of the same diameter as the central body tube will give you an exact fit. To use five larger diameter fins or seven smaller diameter fins will require some experimentation and/or the use of spacer shims to make everything come out right.
Larry Brand has done several year's worth of experimentation with tube fin rockets and generally states that while six tube rockets are the easiest to build there are not as efficient as the seven tube variety. There is much less drag and such rockets have a fairly consistent coefficient of drag across the range of velocities that can be encountered with different motor choices.
An aerodynamicist (is there such a word?) can probably explain this better but I am only a retired telcom guy who builds a lot of tube fin rockets.
EDIT: Forgot to mention that tube fin rockets are extremely stable with respect to wind. Weathercocking and tube fin rockets are things that you almost never hear in the same sentence.
Kit (AKA Cranky Kong)
Total Total Impulse as BAR: 7,753.69 Ns (Equivalent to a 51% M motor.)
=| Calirado, Colofornia...what's the diff anymore? |=