A few questions to give you the best answers:
When are you planning to visit? Conditions vary depending on time of year.
How experienced are you diving cold water, using thick exposure protection (min 7mm wetsuit)?
Will the 3 of you be buddies? What are the other's experience levels?
Will you have gear or need to rent?
I ask because you can do this several ways and get the Catalina experience.
1- Charters from Long Beach/San Pedro- If you are looking to dive on a weekend day then you should have your choice of several dive boats that do 3 tank single day trips. Cost is $110-$150 but includes air fills, food, etc. But our dive boats do not put DM's in the water. You are on your own to plan your dive and dive your plan. Depending on conditions, and if there are students from local shops on the boat, you may dive sites that are somewhat easy, to advanced. The cost is not bad when you consider the Catalina Express (Next option). Look at the sticky post at the top of this forum (Catalina Island Info) for boat recommendations. I know that you can rent gear from Sundiver right at the boat. Most have tanks and weights (make sure you verify).
2- Catalina Express to Casino Point- If you have your gear or want to rent on the island, you can take the Catalina Express over and dive Casino Point (known as the dive park). There is a protected area for divers with a stairway entry/exit, and on weekends a small trailer to do air fills or swap tanks. Catalina Divers Supply, and Scuba Luv can outfit you if you need gear. You can also talk with them about hiring a DM if you need one. The Express is about $76 round trip, plus parking, food, tanks and fills. When compared to a dive boat, it is almost the same cost.
I suggest reading the ongoing post for more details
Catalina Island Info