yup 3-3 on A320 2-2 on E190's.

SEA Jetblue check-in counter is very small and easy to use..since there's usually only one or two flights checking in at a time- there should always be an agent able to help you. there are 2 self-check kiosks..but there's really no point in using it if you have bags. mostly people w/o bags use the kiosk though people with bags use it too..but eventually they have to deal with an agent anyways.

as for sitting together. i would give Jetblue a call and see what they can do for you.

you just need to be at the airport about 2hrs before and it should be ok. the gate used in SEA is A10 which is quite a long stroll through the A terminal so be prepared for that. The moving walkalators start about halfway to the gate. Otherwise everything should be quite efficient.