Excel formula skip rows help!

Hello, I am having trouble copying something with formula. I have a 2 sheets in the same work book. One of them is the weighted average. My formula works wells just when i try to drag it down it does not come out they way i need it too.

This is the formula in C1
= IF('Weighted Average'!M1 = "",A1+B1,IF( 'Weighted Average'!M1>0.001,'Weighted Average'!M1))

On the weighted average page where i'm pulling the value of M1. It has to skips two cell and the next value would be M4.

So cells C1 to C3 would have these formulas in them.

= IF('Weighted Average'!M1 = "",A1+B1,IF( 'Weighted Average'!M1>0.001,'Weighted Average'!M1))
= IF('Weighted Average'!M4 = "",A2+B2,IF( 'Weighted Average'!M4>0.001,'Weighted Average'!M4))
= IF('Weighted Average'!M7 = "",A3+B3,IF( 'Weighted Average'!M7>0.001,'Weighted Average'!M7))

However, here is the problem when i select cells C1 to C3 and drag it down to C400 the sequence is messed up.

Here is how C1 to C6 will look if i drag it down.

= IF('Weighted Average'!M1 = "",A1+B1,IF( 'Weighted Average'!M1>0.001,'Weighted Average'!M1))
= IF('Weighted Average'!M4 = "",A2+B2,IF( 'Weighted Average'!M4>0.001,'Weighted Average'!M4))
= IF('Weighted Average'!M7 = "",A3+B3,IF( 'Weighted Average'!M7>0.001,'Weighted Average'!M7))
= IF('Weighted Average'!M4 = "",A4+B4,IF( 'Weighted Average'!M4>0.001,'Weighted Average'!M4))
= IF('Weighted Average'!M7 = "",A5+B5,IF( 'Weighted Average'!M7>0.001,'Weighted Average'!M7))
= IF('Weighted Average'!M8 = "",A6+B6,IF( 'Weighted Average'!M8>0.001,'Weighted Average'!M8))

As you can see the A and B cells copy just fine in a sequence. But the M cells go out of wack. Is there any way to do this without manually changing each M?
I just want the value to add 3 to every M. As in, M1, M4, M7, M10, M13....

Here is an attachment of what I am trying to do
Exceltest.xlsx