Hi Ankush, My suggestion would be to setup base at Kasol with Om Negi's camp. It's a beautiful place surrounded by mountains and on the banks of river Parvati.
One of the days you can go to Malana and another day you can go to Tosh.
When in Kasol you can also visit Manikaran for the hot springs. Contact Om Negi if you need. He's a helpful guy. P.S. Here is the link: http://www.negis-kasol-malana-parvati.com/
If you can do some small planning at your end, Shimla & kasol trips are not much complex. The only decent( convenient+comfort+economical) mode of travel is via Volvo bus which cost around 1000-1200 per person from Delhi to Shimla and same from shimla & kasol. But if you have only 3 days i'd recommend to travel to Kasol only as it has other tourist destinations as well in its nearby perimeter like Manali , Kullu , Manikaran etc. It'd take 10-12 hr to reach there and you can spend 2 days to explore the places. For detailed info, fell free to messge me.