@kworld12 At three she's old enough to understand that things are going to be different and to talk about her new sleep routine, and even to make some choices about it (like teeth first or potty first etc) - you could make up a chart together with your routine.
Then give her a few days warning that things are changing. Then go ahead and make the change, and keep the boundary firm (you're also in a big boundary testing age! Keeping boundaries firm helps kids feel secure.)
I reckon the excuses method might work better for your kiddo- the first one I'd make super short, like "I'm just checking your door closes properly".
Are you fans of Bluey? Even just as a parent I'd give Sleepytime a watch (Bingo talks about having a big girl sleep and waking up in her own bed) (also, emotional, bring tissues.)
For older kids, there are four methods that I know of.
1) chair method- also called sleep lady shuffle. Basically you gradually move a chair out of the room by moving it a little each night.
2) excuses method. This is like reverse Ferber. Instead of check ins, check out. Do your routine with snuggles etc, but leave with an excuse like "I have to go to the toilet, I'll be right back" etc. Teaching kiddo that you do keep your promise, you will be back, and they are ok to be alone and hopefully fall asleep. Longer and longer "check outs" after a fairly short initial one.
3)
Super Nanny method. There's a process, but basically you just end up plopping kid back into bed until they give up and stay in bed.
4) baby gate the room, kid proof, they may fall asleep on the floor, that's cool. Then do CIO or other more standard younger kid method. (This is for kids who aren't in a cot/crib anymore. )