Last 3 months accumulate values

[#1]

Hi Everyone,

I need your help. I need to calculate values based on the last 3 months. My dataset doesn't contain values for all months. Please take a look into table below.

Problem in Accum by last 3 month. I'm not sure if I can use Rank function for it.

Year_Month	Value	Accum	Accum by last 3 month	Accum by last 3 month (Formula)
2012.01	100	100	100	2012.01
2012.02	150	250	250	2012.02 + 2012.01
2012.03	50	300	300	2012.03 + 2012.02 + 2012.01
2012.06	300	600	300	2012.06 + 2012.05 + 2012.04
2012.08	200	800	500	2012.08 + 2012.07 + 2012.06
2012.09	150	950	350	2012.09 + 2012.08 + 2012.07
2012.10	50	1000	400	2012.10 + 2012.09 + 2012.08
2012.12	300	1300	350	2012.12 + 2012.11 + 2012.10

Thanks in advance for your help!

[giakoum]

you can use Above function to achieve that.