How to Calculate the Last three Months Sales

[#1]

Could you help to tell me how to write the express to Calculate the Last three Months sales?

For example:

The 2016/1 sales amount is 100

The 2015/12 sales amount is 200

The 2015/11 sales amount is 300

The 2015/10 sales amount is 400

Select  (2016 -1)

The Result:

Year   Month   Sales

2016   1            100+200+300

Select  (2015 -12)

The Result:

Year   Month   Sales

2015   12           200+300+400

[#2]

Hi andrii smirkin,

I use your solution, it wroks fine now. Thanks a lot.