Statistical technique to compare more than one groups with given constant values

Hi! I have three samples (S1, S2, S3) to test for nitrogen content and three methods of analysis (M1, M2, M3). I am trying to find out the most accurate method of analysis. The methods will provide with me mean nitrogen content values for different sample. Since the samples are standards, I also have the correct nitrogen content for all of these samples. To find out the most accurate method, I have to compare the estimated nitrogen contents with the real ones.
If I only had one kind of sample and one kind of technique, I would have done a t-test to compare the estimated with given constant. Now, since I have many kinds of samples and many techniques, can you suggest me another statistical technique to do this analysis?