what is the best clothes to wear in beijing in the month of march?

Hi, at that time, it is still cold, so you need prepare some warm clothes.
For details, a sweater and a thick jacket are enough in early March. Taking an extra outer layer, gloves and hat as well in case you encounter any unexpected strong winds and a sharp fall in temperature.
If coming in late March, a thin sweater plus a light windproof jacket will be adequate.