Butt implant keeps swelling on left side 3 months after surgery.

Allow me to answer your question as best i can without any photos. More importantly, please keep in mind that an in-person consultation with physical exam is first necessary before any legitimate evaluation or final recommendations can be made.   That being said, it sounds as though your doctor has thus far done the right thing investigating for a fluid collection (a.k.a. seroma). Couple questions, what exactly did your doctor do to investigate for a seroma? Did s/he try to aspirate fluid and/or did s/he perform any radiologic examinations such as ultrasound or MRI scan?   What type of implant did you have placed, is it a round implant or possibly an old oval-type teardrop shaped implant? If it is a round implant and causing these problems, possibly it is shifting within the pocket.  If it is the old teardrop shaped type implant then it very well could be rotating clockwise or counterclockwise thus causing the illusion of swelling. Glad to help. 

This is a bit tricky. Your surgeon has done the right thing....first check for fluid collection that is the most frequent problem. If negative, next step should be to check your labs and screen for a discrete,subclinical infection. This is rare, but possible.I suspect that this finding might also be negative, in this case the the best action for the time being might be no action. From a surgical perspective, three months is still a relatively short time. There is potential for self limitation of the problem, in other words there is a good chance it will just disappear on its own.Imagine it this way: especially when the implants are inside the muscle (and this is where they should be) the muscle needs some time to adapt to the new situation. He usually does that within 4-6 weeks but it may very well take longer.So for the time being. Don't stress, observe and wait till 6 weeks post op. If the problem is still present then you and your surgeons can come to more robust conclusions, one way or the other.

You most likely have a seroma on that side which is just a build up of excess fluid. This will need to be aspirated to avoid further complications. Seek consultation with a plastic surgeon in your area to address this ASAP.