Anyways, the main component you should focus on is the GPU. While the 7990 is the fastest in terms f hash rate/card, it is too expensive and as a result not the best in terms of Rs./khash.
For 3L, you can build 3 rigs (unless you can get your hands on some powered risers) - Config:
GPU: R9 280x ~23-25k (gives you about ~700 kh/s) 2-3 on each rig
MB: Gigabye UD3 ~9,500
CPU: AMD Sempron 145 ~1,800
RAM: 2GB Kingston/Corsair ~2,200
PSU: Seasonic/Corsair/CoolerMaster
650w ~3.5-4,000 - will barely take 2 280x's (i wouldnt risk it)
850w ~9-11,000 - will take 2 280x's
1000w ~ 12.5-14,000 - will take 3 280x's
For the PSU, you can choose to go with multiple 650w's on each rig in order to reduce cost. Use a 2PSU cable.
corsair GS800 - 800W power supply only about Rs. 7,500 ,