Can I do this?

[dws]

I have created a user input form with three input fields [a], and [c]. I want the user to populate any two of the three fields then have the third field be caculated based on the user's input [c]=[a]-, [a]=-[c] and =[a]-[c]. I would love to not have the form validate until two of the above fields have been populated. I would then like all three values to be stored in a table. Can this be done?

Thanks
Could head me in the right direction?

 

[FoFa]

yes, using VBA to do the checking because what if they enter all 3 as an example, or only one?
A simplistic way of visualizing this would be something like this:
IF A is null goto calcA
IF B is null goto calcB
IF C is null goto calcC
IF YOU GOT HERE, THEY ENTERED 3, GIVE AN ERROR
calcA:
IF B is null or C is NULL THEN Give ERROR, have to enter two, exit
A = B-C
Exit
calcB/calcC:
Same as A basically