3 phase is a completely different animal then single phase. You have way more power then you think. At 150 amps, you have a lot more power availbe then a 200 amp single phase. A 200 amp, 240 volt sevice is 48 KVA, a 150 amp at 208 volts is 54 KV because it has a 1.73 factor to multiply it by for the third leg. Voltage on a 3 phase is going to depend on your transformer connection on the secondary side. Most likely you have a 120/208 volt service. This is called a "wye" connection. You could have a delta or open delta wich is a little more tricky is the sense you need to be more cautious in your 120 volt connect since you have only 2 of the three legs to take 120 volt fron. The other leg, often called the high or red leg has 208 volts to ground and will fry your stuff! If you have this connection, you should have a second panel dedicated to 120 volt circuit use. On the 120/208 volt, wye system it doesn't matter what leg you use for 120 volts as they all are 120 to neutral.
Usually small buidings with light loads will get the open delta system because the utility company can use 2 transformers instead of 3. If you have a 120/208 volt service, you need to be aware that any electric motors need to be rated for 208 volts and not just for 240 volts. I agree, this is a job for a pro to do!