I’m not a big fan of 3x3 bases, I feel they are too small and there is no way to adequately protect your cupboard with one. That said, I’ve made several very small bases that never got raided, you have to use your brains a bit and fool the raiders
If you insist on small, I think a 4x4 or 5x5 but with a mostly empty inside is pretty good. Put some furnaces and crates with random crap at the bottom, sleeping bag, etc and make it look like it’s a noob/abandoned house. Make your forge room on the 2nd floor but don’t keep too much stuff there, just what you’re smelting at the moment and move everything up when you’re done.
Up top, make it at least 3 stories of floors, armored ideally or stone if you can’t armor. Make that first “top floor” at least 3-4 stories high… Feel free to use whatever to close the “empty” sides, it’s kind of irrelevant… I like sheet, it’s cheaper than everything else.
Your bottom (ground) floor, have fun closing it off with 1x1’s or blocks all over the place, make them waste their c4 looking for stuff.
Put cupboards everywhere… and make sure your main is sealed off aggressively to avoid rocket fishing.
Feel free to leave some parts of the bottom floor opened if you get raided, or any other damage that’s done. It’ll look abandoned and raided, won’t be a target for most people.
I’m not spilling all my beans, but there are lots of other neat tricks. People finding my loot room are not really finding my loot room… They’re just being trolled into thinking they did so they gtfo with a sad face. Yeah they’ll get some stone, wood, metal, guns, etc… But that’s just the consolation prize, the real stuff is in places they’ll never expect unless they’re really wise