There is 3 ways:
1) Signup info must be for each of them. In that case signup info will be stored in database or sended via email.
For example you can put them under each other, and user will choose on what term he want to subscribe
2) If you don`t need his info, you can create 3 forms with just Payment field, change their submit buttons value and put them near each other.
3) We can customize PHPForms for you, so you can be able to create forms with many Payment fields on it. It will be looking as you want - one form, signup info fields, submit button and 3 (or more) choices of subscription term. In that case you can send us message from our site
http://phpforms.net/support.html. We will answer you as soon as possible