We rent land and the owner of the land and myself have signed/date a 3 year contract. Is this legal? Can owner changed contract?

If you have a valid contract, the owner can't change any of the terms without your consent  unless the contract provides that he can.   Thus, for example, if the contract provided that you can farm such acres as the owner shall designate, he could change the acreage.  If there is no provision giving him the right to change the terms, however, then he cannot legally do so. I see no reason in your email why your contract should not be valid, but the law may be different in North Dakota.  Your contract is in writing and signed, and in the states in which I practice would not require any further formality to be valid.  Again, it is possible that North Dakota law would require that the contract be witnessed, notarizede, or some other formality, but I would doubt it.