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I've have two one year old red ear sliders and have recently bought a new baby red ear slider. I want to put it in with the other two turtles but I don't know if I should. Will they bully the newcomer or worse try to eat it? My new baby's shell is still kinda soft so I really need advice. Right now the baby is in a seperate container. My baby is also floating, she can go under water but she floats like most of the time. Is this normal? Is the floating because she's still a baby and light? Or does she just prefer it? Please help me.

First of all, turtles of significantly different sizes should not be placed together in the same tank. They should be kept separate until the smallest one is closer to the size of the others. AND, they should be in an appropriately-sized tank (ideally think of 10 gallons of tank per inch of shell length for each turtle).

Second of all, any "new" turtle should be quarantined in its own setup for at least 90 days before it is placed in an established tank. This is to ensure that the new turtle doesn't have anything that the other(s) could catch. It is not foolproof, but it certainly helps. If the baby can't submerge and is just floating, it should be a red flag to you to keep it separate from the others. It will need the same equipment that would be in a regular tank (heat, light, basking area, etc.), otherwise, if it is sick, it will become even sicker.

Lastly, how big is the tank that the other two are already occupying (in gallons)? If the tank size is inadequate, chances of bullying and worse aggression are greatly intensified.

ditto on what Moderator marisa posted.
Even if your turtles don't bully the baby or try to eat him, they may nip at him out of curiosity. The baby might even get stressed in a new environment with the big guys. Stress won't be good for a baby whose developing.
And in the future, if you do try to put them together, your older turtles might get territorial so have them meet on "neutral" ground first and watch them CAREFULLY.