Previous Month Value How to show next three Month

[Anonymous]

 i have table data Month Wise .  if user select July(selected Month hidden in Graph) Previous three Month value  multile by contant value 0.2,0.3,0.5

april 65.01 * 0.2 = 13.02

May 65.03 * 0.3 = 19.59

June 64.02 * 0.2 = 32.01

Toal avg value = 64.71  this value i want show after  selected Month like August,Sep,oct 
August        64.71

Sep             64.71

Oct             64.71

after that  November it will increase by 1 and december it will increase by 2 

November  64.71+1=65.71

December  64.71+2=66.71

How to work on it dax. i am new to dax functions .any  idea ..thanks in advance 

[amitchandak]

@Anonymous , Try with help from date table

var _max = MAXX(allselected('Date1'),'Date1' [Month])
var _this = month(_max)
var _last = month(eomonth(_max,-1))
var _MTD = CALCULATE(SUM(Sales[Sales Amount]),DATESMTD('Date'[Date]))
var _LMTD = CALCULATE(SUM(Sales[Sales Amount]),DATESMTD(dateadd('Date'[Date],-1,MONTH)))
var _2LMTD = CALCULATE(SUM(Sales[Sales Amount]),DATESMTD(dateadd('Date'[Date],-2,MONTH)))
var _3LMTD = CALCULATE(SUM(Sales[Sales Amount]),DATESMTD(dateadd('Date'[Date],-2,MONTH)))
return
Switch ( True() ,
_this =7, _3LMTD *.2 + _2LMTD *.3 + _LMTD *.2 ,
//Add other logics
//else if need
)