Hello korweabo0,
Interesting setup you are creating. After reading your recent post I see why you are connecting them. If I were you I would draw the 15/30 gal tank the same height as the 20 gal tanks for clarity. Without the dimensions I was wondering how you would pull this off.
Water weighs 8.35 lbs/gal so:
20+20+15=55 gals * 8.35 = 459 lbs + gravel, decorations, etc...
and
20+20+30=70 gals * 8.35 = 584.5 lbs + gravel, decorations, etc...
I don't know how much help I can be on the stand but if I was building one for this setup I would definitely go with at least 2x4's for legs and 1/2" - 3/4" piece of wood on top depending on what you are using (plywood or pine or ???). You wnat to brace the legs at the top and bottom for best results. Someone else may help better with the stand design.
I have never connected fish tanks together with pvc but have connected 55 gal barrels and 275 gal water totes together for pond filtration using 3" and 4" electrical connections like shown here in post #2
Birdman's Fluid Bed Filter - Koiphen.com
You can use the male adapter on the inside of the tank and cut off the "slip" portion that fits PVC pipe like in the link above. The male and female electrical connectors screw all the way together unlike standard pvc fittings and I used this to seal them
PL Sealants & Adhesives
This product is black in color so clean with mineral spirits and use gloves for easy cleanup.
If you want the passways a certain color you can paint them with Krylon Fusion spray paint which I believe is fish safe.
The connectors are @ $5 each
Are your tanks glass or acrylic?
Do you already have 3 canister filters?
Hope this helps.