Driving to Dover (and back?) will take up a reasonable chunk of your 3 hours. If you don’t mind staying inland you could go up to the top of the Devils Kneading Trough above Wye for an amazing view that includes some sea, and then head to the Five Bells in Brabourne for food (get the triple-cooked chips!).
If you’re on the way to Dover for the ferry anyway, then Samphire Hoe or the National Trust White Cliffs of Dover are good for walks. Kingsdown/St Margaret’s are fairly nice villages with reasonable, if stony, beaches. Dover does have a beach right in the town, but it’s a bit of a hole. Haven’t spent much time in Hythe/Folkestone, neither have great reputations, although Hythe to Dymchurch railway is meant to be nice.
Probably too far to travel, but Broadstairs is good for the proper seaside experience, some lovely sandy beaches around, and a pretty seaside town.