If can we adjust of igst input to cgst and sgst

We got the vat dealer registration on recently, we didn't apply the CST registration.(1) In the GST act, can we buy the materials from inter state.(2) if we buy the materials from outside state, can we adjust of IGST input to CGST and SGST. (3) Some of them have said, without CST regsitration, cannot be do the interstate transactions in GST act and cannot avail the IGST input. Please let you clarify my Query ?