Hi, L.,
It is unusual for a 3.5 year old to start getting loose teeth, but there is a great deal of variation in this process. Is there any pain or bleeding associated with the loose teeth? Which front teeth are they? Do they move more than one millimeter in each direction at the tip ? This may be hard to estimate, but this can indicate how much of the root is left. Baby teeth don't become loose unless there is a new permanent tooth underneath in the bone pushing against the root and causing it to resorb (melt away). The teeth may even tighten up if the permanent teeth stop moving for a bit (sometimes this happens). If you are really concerned, you could have your dentist take an X-Ray to see what is going on inside the bone, but it sounds like a normal, if early, process.
If you have more questions, you can call me...D. Cotner, DDS ###-###-####