The Battle Tent...A Mini Battle Frontier In Hoenn, there are 3 of them that exist and they all share styles of battling with the Battle Frontier. Here is a list of the Battle Tents & What they do. Each of them let you have 3 Battles in them before letting you out

Slateport City

The Slateport City Battle Tent is the first one you will come across. This Battle Tent's Style is identical to that of the Battle Factory In which you have to pick 3 Rental Pokémon out of 6 Random ones and use them to battle
And Once you have beaten the battle, you can get your pick of one of your opponent's Rental Pokémon...letting you improve your team

Verdanturf Town

Verdanturf town holds the second Battle Tent, it's based on a Battle Frontier bit which I do not know about, probably the Pyramid or the Hut and in this. In this you use your own Pokémon (barring the legendaries) and the opponents match your highest level. So you get your basic Battles...
...But they aren't exactly Basic. When you go to fight, you can only use one attack and that attack is chosen at random. You need luck or a very strong attacking Pokémon to win this one

Fallarbor Town

Fallarbor Town is the last Battle Tent you will encounter. It is identical in style to the Battle Arena In which you use your own Pokémon, enemies match your highest level. In it you battle like normal...
...Unless you don't faint the Pokémon in 3 moves. If you don't then you're judged on the success of the attack. Triangle is 1 Point. Circle is 2 and Cross is 0. The most points win