Verizon will provide the best nationwide coverage around the USA with AT&T catching up in second place. In your situation I would buy a no contract flip phone from Verizon. The flip phones can go weeks without recharging. My Blackberry needed daily charging and I find a flip phone a lot more convenient.
Verizon has prepaid plans where you get a phone and pay a $35 activation fee for $60 and then about a dollar a day for service with no contract required. Figure $100 for the first 30 days and then $40 a month for the remainder of your trip. For the 3 months the total cost will under $200, which for the USA is not all that bad. With the US companies you will be charged for inbound calls as well as outbound ones so buying an unlimited minutes plan is usually a good idea.