Is there a brush cutter that will do 3" brush with a pto power at 25hp..?
Do it for how long and for how fast?
Trust me on this, you don't want to be actively cutting 3" dia stuff. I run up against some big stuff with my NX5510 and a BH 286 (considered "medium duty" - 90hp gear box and extra deck plate welded on it [because the previous owner used it commercially to chew rocks! it's a good 800 lbs.]) and the noise that this combination makes can be horrendous. Further, how are people figuring their sizing, is it like they've got a plantation of saplings or something? My property has tons of different stuff, I cannot gauge what exactly I'm going to hit (esp when lots of stuff is hidden). I know, however, that when the racket becomes too much for my nerves it's time to stop cutting whatever. And note that my NX is a cabbed tractor.
As others have noted, the material in question, be it grass or what have you, makes a big difference. My B7800 with an honest-to-god "light duty" 5' cutter (Rankin) spent years cutting some really tall grass, some over 10' tall (Reed Canary grass) as well as slamming into all sorts of woody debris, all in the quest to displace invasive blackberry with [non Reed Canary] grass. That 10' grass work was SLOW. BUT, 3' grass down at a neighbor's was WAY slower! Not sure what it is, but it's ugly thick stuff (fortunately I've got none of that on my property). But getting back to the '3" diameter" stuff, which grass isn't, but I figure saplings and tree limbs/debris is, well, kind of also depends on what kind of material. Hard stuff, or older decaying/punky stuff? Makes a big difference. Again, I don't think anyone would last too long chewing through 3" dia material. Folks who do roadside maintenance chew through some big stuff, but they're in BIG tractors and they generally are running flails (which are a lot safer when around the public).
I can state factually that my B7800 (PTO maybe 23hp) and the aforementioned 5' cutter cut through stuff much bigger than 3" dia. That combo has sliced through steel wheels and other car parts (favorite was a battle with an old VW transaxle- happened at dusk and it was quite the light show, sparks flying all over the place- no winner and no loser, but I called That bout early!). Now then, if I had to cut stuff like this for money, on a day-in and day-out basis, there is ZERO way I'd expect this to work out. My NX and it's RC is a far better match and I wouldn't think of trying to do that with it either.
RCs are mostly for brush, lighter stuff. They are OK for grass. They're a general purpose mower. Battles against big stuff starts to put the PTO on your tractor at risk: while shear pins and slip clutches are meant to protect your tractor's PTO it's not a guarantee). Rent something more appropriate (industrial) and spare your smaller equipment (which you would rely on to do the majority of your work, an amount that will never, given any machine you get, manage to do 100% of what you need- one can always find something that is too much for what you have!).
If you have a bunch of big stuff to cut then look to get a more suitable machine (perhaps a skid steer with an RC).