His annual salary for the first year of working is 6000 x 12= $<<6000*12=72000>>72000.
Kirt's monthly increase is $6000 x 30/100= $<<6000*30/100=1800>>1800.
So, he received $6000 + $1800= $<<6000+1800=7800>>7800 monthly on the second and third year of working.
Kirt's annual salary in the second and third year was $7800 x 12 = $93600.

Thus, his total earning after 3 years is $72000 + $93600 + $93600 = $<<72000+93600+93600=259200>>259200.
#### 259200