You have a corner grounded delta three-phase setup
While there's only one way to wire a split-phase service drop correctly, with the neutral point smack in the middle, with a three phase service, there's no clear place to put the bonding point that designates where neutral is, especially if the power company's transformers are wired in delta instead of the more modern/common wye configuration, considering that unless you are dealing with a continuous industrial process where uncontrolled breaker tripping could damage more than it saves, the NEC would really rather you not let your shop's electrical system float off to an obscene voltage relative to the earth, as that can cause insulation damage, shorts, sparks, smoke, and fires.
One system, called high leg delta, put the neutral point on a center tap of one of the windings. This had the advantage of providing 120V for lights and outlets; however, it came at a price, namely that the phase opposite the center tap shot up to 208V with regards to the neutral wire, rendering it useless for anything other than three-phase power. High leg systems also require a three-phase, four-wire panel, yet waste a significant number of spaces in that panel due to the B-phase being too much for most light-duty circuit breakers to handle.
Whoever had the utility run three-phase to your shop went the other way, though, and hooked the ground bond up to one of the incoming phases from the utility (typically the B-phase), yielding what you have, namely a corner grounded delta service. This provides a single voltage (240V in your case) with two hot phases, a grounded phase, and a ground (EGC). However, while you might think that single phase panels work with this, the matching breakers often are no good for you (your choices are QO...or heavier duty stuff rated for 277/480V systems), as corner-grounded delta is quite hard on breakers, requiring them to be specially rated for use in that system.
For 240V single phase loads, you can use this, but be careful!
If you have single phase loads that accept 240VAC (such as universal or 240V ballasts, or universal switching supplies), you can wire them line-to-grounded-leg (or perhaps even line-to-line) from your panel; however, there are a couple of caveats to this. First off, since you have a true three-phase panel on this system, you'll be using individual two-pole breakers for each circuit. Second, and more importantly, some loads are only rated for 150V-to-ground or less, and your system supplies 240V-to-ground, so you'll need to pay careful attention to the specifications of things; in general, anything rated for 277V maximum is safe, though.
For plug-in loads, furthermore, you'll need to use the correct receptacle for the task, too, most likely a NEMA 6 although one pole of the receptacle may wind up on the grounded phase. Finally, you won't be able to find GFCIs for this system, since GFCI receptacles are 120V-only contrivances, and GFCI breakers are not rated for 240V-to-ground service either.