The process Keet describes gives you perfect round holes. But... it's a bit fiddly.

If a slightly less perfect result is acceptable, there is a simpler way.

See the attached file which should explain the process.

Instead of thick walls (for clarity) use the thinnest walls possible, e.g. 0.1 mm. If necessary adjust the created floors to eliminate the thin (0.1 mm wide) gap between the two rooms.

Again, it's not as perfect as Keet's method, but for me it's good enough and it can all be done within SH3D.

H