Having a c-section for placenta previa. was told to get to hospital 3 hours ahead to get prep for surgery. what will they do? what takes so long?

It always takes time: Any planned surgery takes time: there's paperwork, a pre-anesthesia evaluation, a monitor strip of your baby's heartbeat, prepping the site of surgery, etc. In addition, you must remember that with ob, you may be at the "mercy" of other situations with greater urgency. For example, if the need for an emergency c-section arises for another patient, they'll go first - as is appropriate. Good luck.