Calculate a Field Value in Access

[m3trj]

I have this Access database, and I want the contents of a particular field to consist of a calculation using values from another table.

I have table1, which contains the field with the calculation.
Table2, contains one of the fields containing the data to be calculated.
Table3, contains the other field with data to be calculated.

So I want a field in table1 to be the value in the field in table2 multiplied by the value in the field in table3.

Does anyone know how to do this? I'm guessing I need to enter some kind of special equation in the field in table 1?

Hope that makes sense!

Thanks in advance.