As you are staying for 12 days only I would suggest you choose two places only and not three. To move every 4 days will mean a lot of packing, unpacking and time spent checking in and checking out of different accommodation.

I would suggest six nights in Ubud and six nights in on of the beach areas at either Sanur, Seminyak, Kerobokan or Cenggu. The last three places are all near each other with Sanur being a bit further away from these (about a 15-20 minute drive).

As Bali is a relatively small island you can easily go on day trips and excursions to visit temples, historic and ancient sites and enjoy the beautiful natural scenery.

It may be a good idea to go to your local travel agent and book a package holiday with accommodation (usually includes breakfast) and transfers all in one. Much easier than working it all out yourself.
Travel brochures will give you many accommodation choices and then you can work to your budget. Do you have a Flight Centre travel agent near you?

Not knowing how much you would like to spend on a hotel it is difficult to make any recommendations.

Perhaps if you find some places that you like you could ask for opinions on this forum and advice from anyone who has already stayed there.

Good luck with your plans. I'm assuming it is your own wedding so congratulations.

Catherine