My Ezzy 100% carbon mast is starting to get some cracks in the mast tip about 3 cm long. I am wondering if I can saw off the top 3cm and re-attach the plastic thingy, would this prevent it from getting any worse?

Ive done this in the past but not with 100% carbon masts. I always coated the new section with a thin layer of epoxy to seal the section.
If its a SDM then you will probably have to pack the mast tip/plug with tape to ensure a tight fit. Otherwise it will stay with your sail on the mast cap in the sail.
Dont have any experience with RDM's, sorry.
Obviously cutting 3 to 4cm's of the tip will affect your extension settings.
Hope this helps.

I'd go with landyachts suggestion.

If the cracks are all the way thru (not just surface) drill a 1mm hole at the end of each, stops them going any further.
Sand it well, then go around with at least 3 layers of fibreglass and epoxy like West 105resin and 206 harderner.