Does Free Range vs High Fence matter to you or your hunting companions? If you're OK hunting HF, how small of a property is too small? Do you care about supplementation of animals? What about put & take? I'm not knocking HF hunting, there are some great HF Operations. You just need to know and understand your limits as well as the parameters that you want, then communicate those to the outfitter/ask them.
As for your questions;
3 Hunters - there are outfitters who could accommodate 3:1 hunting, depending on each persons hunting objectives and the length of the hunt this will determine how feasible it is. Personally I'd suggest a 2:1 and 1:1. You can all switch who's alone and together different days and I'm sure there will be a day or two you could all go 3:1 or 3:2 if you want.
$30k budget is feasible it depends on what your expectations are, how much you plan to taxidermy/ship, class of airfare, and game targeted.
Namibia vs South Africa; that really depends on the outfitter selected, you wants/desires, and the species you wish to target. I too suggest central Namibia, or the eastern cape of South Africa. I also had a great hunt in Limpopo area with
@MATTANJA HUNTING SAFARIS
Renting vs Own Guns; I suggest taking your own firearms. The process isn't hard and it gives you some great memories with them.
Target Species; Personally I like to hunt animals where they're endemic and naturally occurring but don't knock anyone for hunting something that's introduced. It's your hunt so enjoy it and hunt what you want/how you want. My suggestion would be targeting 1 or 2 bigger/costlier animal (Kudu, Eland, Nyala, Waterbuck, Bushbuck), 1 or 2 mid-priced animals (Zebra, Wildebeest, Gemsbok, Hartebeest), and 2 to 3 smaller/lower priced animals (Impala, Warthog, Blesbok, Springbok, Duiker, Steenbok, etc). Location will somewhat dictate price on them and sometimes a mid-priced animal might be in the lower pricing category, or vice versa. Example Hartebeest and Gemsbok are often much cheaper in Namibia where they're endemic vs South Africa. Springbok in Limpopo will be a lot more then in the Karoo or Kalahari. Pick an animal or two that are your "Must Haves" then enjoy the hunt and see what you see.
I noticed you want a Waterbuck, in my opinion they're one of the most underrated Plains Game animals to hunt. Good luck