There are 3 different numbers to look at before determining an asking price for your home:
1- The fair market value of your house and land. I'd suggest getting 3 CMAs or appraisals from 3 different companies and take the highest estimate.
2- What it's worth to the developers. Find out what they are paying per lot for the rest of the subdivision. This should be public record, ask at the city or county for sales and tax records. You have 3 lots in your acre. This is just a starting point. Your land may be worth more than the land price to them, because their development will be more marketable, easier to subdivide, more cohesive, and simpler for construction if they don't have to work around your house and family.
3- What it's worth to you. Start looking at homes, talk to realtors and determine how much it will cost you to move. Not only that, but consider the time and effort involved in moving. If these developers want you to move, they should be willing make it worth your while, and the move should put you in a better, not worse financial position than you are in now.
Don't go into this alone. Find professionals to help you and even be willing to pay for good advice. Your home is probably your biggest investment and the money you get out of it can vary by 10's of thousands of dollars depending on how knowledgeable you are and the professional support you have.