Average of last 12 months

Hi guys, I can't think of a way to explain this in short, please have a look at the attached spreadsheet.

I've got a table on the right with monthly values in multiple years.
I've got a top left 'table', where I'll change the month & year to the current month.
I've got a bottom left 'table', where I want the 3 values mentioned to show up.

1. I don't know how to get the value for C16 using a formula. Is there a simple way of doing it?

2. You know how in formulas such as Countif & sumif you can put the condition as "<4", but not "<C3" (referencing a cell). Is there a way to put the condition as "<C3", where C3 is the value of the cell C3?

Cheers