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Here's a easy method to get 3G speed in 2G pack in docomo.
Activate 3G on your docomo card, skip if already activate. Now subscribe to Rs.15 2G plan. Don't subscribe to any other plan like Rs.45 plan etc. Select UMTS mode in network settings and enjoy 3G speed in 2G plan. If this doesn't work, you can try on a new card as your old card may have got blocked.
I can not guarantee that it would work because I have not tried this personally.