It is possible, but probably more work than you want to do. To accomplish what you want you will need some means of load balancing, and that means you will need more hardware than just a PC.
Your multiple USB dongles are basically multiple network connections. If you had some more hardware (i.e. a suitable mobile broadband router (acting as modems) for each USB dongle plus an extra router for load balancing) you could make it work.
There is a good tutorial on the subject at Surfboard Hacker (the one I used to make load balancing work for me). Here is a similar tutorial and here are the scripts you will need if you do this with a DD-WRT router. You will have to modify some what they say for your purposes since you are using mobile broadband instead of a standard DSL or Cable modem.