I own/use the BH 300, but have never used either of the other two machines so I won't comment (on them) beyond saying they're pulse machines with no discrimination (that I'm aware of). I believe the pulse machines are a bit deeper, but I'm not sure about that either.
The BH 300 is a hefty machine: not the choice I'd make for hunting dry land for long periods of time. My understanding is that is can be belt-mounted, but I have not done that (and don't plan to). The BH 300 runs two frequencies at the same time (I don't recall the exact frequencies). It is waterproof to 25 feet and has two modes: discriminate and all metal. All metal is the best way to hunt if you want more depth (which is very good on the BH 300). There are 3 lights (sometimes a bit difficult to see while hunting) that give the user a rough idea of what is in the sand. Green is generally a coin, yellow is generally jewelry or trash, and red is generally iron/trash. It is important to get a good ground balance with the machine. Mine has the larger coil (would have to send it in to change coils) and it covers ground quickly. I've had no issues with the coil floating (a common complaint with the older versions) but I don't go beyond calf-deep when I'm in the water.
I've read reports of people having a really hard time getting the BH 300 to ground balance, with some reports saying the machine isn't usable in certain types of soil (heavy minerals, I believe). I have not has an issue with mine (in NJ) being difficult to ground balance. In my somewhat limited use, the BH 300 seems to be pretty sensitive to smaller items, but I can't say how it would do on small items (such as small gold chains, etc..) compared to the other two machines you asked about.
The BH 300 is no longer manufactured by Whites, but I do believe quite a few dealers still have some in stock. Those machines, if new, would come with Whites' 2 year warranty. The BH 300 is made for salt water use - although it would work fine in fresh water too.
I bought mine strictly for those few times I do hunt the shore area (I live in NJ - about 45 minutes from the ocean, and 15 minutes from the bay) and because I'm really hesitant to use one of my other machines in an environment where it will get sprayed with salt water/mist. I don't hunt the shore/bay all that often though, as I much prefer hunting private property (permission) with some history behind it. I do like the BH 300 though, and am glad that I purchased it.
I'm sure someone (with more experience than I have) will tell you more about the BH 300. There are a number of people who use the Tesoro, so I'm sure someone will be able to tell you about that machine.