Hi,

You don't need any tour package for Manali. You can easily explore the area on your own. Let me answer your questions one by one -

1) is early December good time to visit?

Yes, it's a decent time to visit. It might just snow as well. But you should ensure that you carry warm and thick woolens with you. When you reach Manali, buy a pair of traditionally woven woolen socks. They'll really help you bear the cold weather.

2) what all are the places to cover?

In 3 days you must cover the following - Hadimba Temple, Van Vihar, Solang Valley, Jogini Falls, Vashisht, Manali Club House, Local Mall Road, Naggar Castle, Gulaba and the Great Himalayan National Park.

3)any tour guide references?

You don't require any. They'll over charge you and won't show you much.

4) is Shimla nearby?

No. Shimla is at a duration of 10 hours and in a 3 day trip, it won't be possible to cover it.

Or Manali or Shimla better.?

Both places are beautiful and it would be highly unfair to pick just one. If you want to visit both these places on the same trip, invest at least 6 days on the trip.

Happy journey :-)