Cheapest option off the top of my head would be to have a broadband connection and normal phone line coming into one premises, then having a bridged connection to each other premises, so in total you have 3 business hubs, 1 in each premises.
Then, use your 1 free voip line and configure that onto a standard phone in your main premises. And order 2 hosted voip lines at £5 per month each and configure them onto a the two other bus hubs onto a standard phone.
Then you have 1 broadband connection, 3 routers, each transmitting wireless, each with there own voip phone.
If you wanted to go down the PSTN line you will need to install a line in each building (around £100 from memory), each with there own PSTN line (line rental around £12pm) and BB in each (around £20~25pm).
It will only work if the broadband speed is fast enough that is, less then 5 or 6Mb and I wouldnt bother trying it, you will have slow internet and the phones will cut off.
gs
