Like in subject, can i use MK1 speedo in MK3??

yes they are all the same

The Mk3 wheel spindle is bigger than the MK1.
The Mk1 speedo drive has to be drilled out to fit the Mk3

Eddie +1. If you are talking about the speedo drive.

If you mean the speedo array then it will fit but you might have to alter some of the connecting electrics as slightly different looms. Perfectly possible.