planning to visit beijing in 2018, whether on middle of March, early or end of April, please advise

All your three choices are good according to the holiday schedule, for there is no festival during the period.
From Middle Mar. to early Apr. , the temperature may range from 0c to 20C, so you are suggested to take your sweater, jeans and coats.
At the end of Apr., the temperature may rise, and it may range from 10C to 30C, if you will go during this time, you can take some light coats and T-shirts.