I am going on a trip to the US and Mexico, I will be entering and therefore exiting the USA 3 times to Mexico over this 90 day period when finally leaving and continuing down through Central America. Are there any restrictions on how many times you can enter and exit the US within the 90-day period?
Your ESTA is good for two years and multiple entries. There's no limit on how many times you may enter and exit the US during that time, though you're likely to make immigration officers curious about your travel itinerary with so many entries in a short time.
If you enter the US by land, you may need to fill out some paperwork and pay a USD 6 fee at the border. If this happens, you'll be given a green I-94W form. When you depart the US for the last time, you need to make sure you return this form, either to the airline, at a land border crossing, or by mailing it in with proof of your departure (e.g. a boarding pass).