Landlord deception

I rent a commercial building. It is a 3 year lease with an automatic 3 year renewal. I have been there since June 2003. Just last week I found out that I have been paying the gas bill not only for my unit (5000 sqft) but also the space of another tenant and the landlord - another 2600 sqft. I objected in writing and asked that the furnace for the landlord and other tenanct be moved off my meter. My landlord claims I "knew" I was paying their bills. Why would anyone agree to that! There is nothing in my lease about paying for additional tenants! The landlord just wrote me a note saying he will put the gas service in his name and we will all pay him proportionally. I don't think I trust him. He also wants to bill me now for water and sewer which I have never had to pay before.
First of all, is there someway I can recover the money I spent on the gas for their units? Or can I ask that the proportion be adjusted for the next 4 years to make up what I paid. Can he just present me with a paper where he has made changes to my lease and demand I sign it (such as the water, etc).
I've never had a problem with him before, I do like the location...however the other tenant is a jerk....I don't want to be taken advantage of.

[OHlandlord]

I would calculate the sq. ft. of your unit and figure out the percentage of the entire area. (your area divided by [your area and the 2600 ft of theirs]) Then figure up how much you have spent on gas bills since you moved in and multiply by the above percentage. That is how much you should have paid for gas for all this time (give or take a little). Subtract this from the total and see how much you have been overcharged. I'd also call up the gas company and have a rep come out and write a statement that your meter is covering lines for those other two areas. With this proof in hand, I would demand this money back. Either by a lowered guaranteed rent on a new lease (with no change in paying for other utilities) or by suing in small claims court. This is clearly illegal. Most states say that all shared utiltiy arrangements (as in you knew that you were paying for their gas) must be disclosed through lease clause. I know commercial lease do vary from residential, but this still should not be allowed.