Fiaxation of pay due to completion of 18 years

Pay band PB-3 ,7100-37600+ GP 4100
Date of completion of 18 years of continuous service on 09.06.2015
please help me to fixation of pay which is admissible w.e.f 10.06.2015 but allowable w.e.f 01.07.2015 considering pay as on 01.07.2014 Rs.20490.

You forget to mention your Band-Pay. Anyway the process goes like this (1) normal increment of 3% (BandPay+Grade Pay) be added to Band Pay; (2) one additional 3% increment (of new BandPay+Grade Pay); (3) new Grade Pay=4,400. So as to arrive at new Band-Pay & new Grade Pay.

your band pay as on 01-07-2014 = 20490-4100=16390; basic pay =16390+4100=20490 ,if this line is correct ,then
initial pay fixation as on 10-06-2015
pay=16390+0=16390
basic pay=16390+4400=20790
your annual increment on previous scale/post=20490*3%=614.7~620
promotional increment=(20490+620)*3%=21110*3%=633.3~640
final pay fixation as on 01-07-2015=16390+620+640=17650
final basic pay fixation as on 01-07-2015=17650+4400=22050

I may be wrong, but I understand this way —
i) Band Pay + Grade Pay = 16,390+4,100 = 20,490 (as fixed 01-07-2014)
ii) Normal Increment on 01-07-2015: 20,490*3% = 614.70 ~ 620 ==> Band Pay (16,390+620)+Grade Pay(4,100)=17,010+4100=21,110
iii) Non-Functional Increment (18-years) as on 01-07-2015: 21,110*3%=633.30~640
==> Band Pay (17,010+640)+Grade Pay(4,100+300)=17650+4,400 = 22,050.
Though both calculations comes to the same point, but there is a difference in the point of offering Grade Pay enhancement.