You have got two options.
Option one.
If you wish to go in sleeper class book in either of the four trains for 21 May from mumbai to raipur.
12879, 12809, 12151 and 18029.
Chances...
more... of confirmation in 3A is brightest for 12879 as WL3 is likely to come to RAC or confirm.
Option 2
If you are desperate to go in 3A and are ready for a changeover at Nagpur, then your options are
Either take 12105 or 12139 on 21 May upto Nagpur from Mumbai and take 12151 on 22 May from Nagpur to Raipur.
Seats avl
12105 more than 50 in 3A.
12139 37 seats in 3A.
12151 11 seats in 3A.
12105 Vidharbha exp departs Mumbai in the evening at around 1900 h and reaches Nagpur at 0710 h.
12139 sewagram exp departs Mumbai at 1535 h and reaches Nagpur at 0610 h.
12151 Samarsata exp departs from Nagpur at 1020 h and reaches Raipur at 1530 h.
This ticket will have telescopic benefits and enough seats are available in 3A of all the trains.. Its upto you to decide