In The Sims 3, two Sims must have a relationship of friend or higher for one to ask the other to move in.
Have the Sim you want to ask perform friendly socials until “[Friend] thinks [Sim] is being friendly” appears.
The target can then be asked to move in.
Why can’t I ask my Sim to move in?
They have to be “sociable” with each other and you need to get a lot of the other “Asks” out of the way, example Ask about Day, Ask about Book, Ask about Career. Don’t interrupt with a funny or romantic interaction because that means you have to start all over. Also you can’t move in someone if your household is full.
How do I get Sims to move in together?
Learning to Sim: Move Your Sims In Together | The Sims Freeplay
How do you ask a Sim to move in freeplay?
To move a Sim to a different house, tap on the town button at the bottom of your device screen (or the corner if you own a smartphone), this will take you to the town map. If you have built several houses and only have a few Sims (example: five houses and four Sims) then one of the houses will be empty.