multiple entries into U.K.

If you are going to spend 90 days/90 days/90 days it will work fine. It would be a huge hassle if you went back and forth between the UK and the continent the whole time. It would still be doable, theoretically, but it would be up to you to keep careful records of your dates to prove you didn't exceed the limits.

Yes, doable and your US passport will allow you to do this with no problems. With that said, however, and I don't know if this is possible, but have the immigration folks at your first destination on the continent give you the necessary date for leaving and when you can reenter from the UK. So, ask them when do I need to leave the continent and when is the earliest date I could return. I don't know, but just counting out 90-days makes me nervous. I'd get a more official count if possible.

Maryam, asking such a question would seem to raise red flags with the immigration people, and they don't know if/when you might be coming and going from Europe in the meantime. If the OP has a concern about the exact date he should just mark the date on his calendar and maybe leave a day or two sooner just to be sure.

I agree, don't ask for a date from immigration officials. I also agree that multiple trips back and forth on a regular basis may raise some flags if you're just there as a tourist. And it will make it much harder to track your days. That said, going back and forth is common and not a big deal, so doing so a few times wouldn't be an issue.

Hi, I did this several times while studying in the UK last year. I would keep exact dates of when you left (every time) and the periods that you stayed. I entered the UK several times in several different locations. Most of the time, the border control agents just asked a few questions, but once or twice, they became really agitated and asked me a lot of questions about what I was doing hopping around so much (as an American student/tourist). Sherry