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?

I've never had a mast starting to split around the tip mainly around the join(especially North masts) and base, however I have shortened plenty of masts by trimming of upto 10 cm of tops and bottoms without any problems as long as you have not made significant changes to the diameter.

cheers.

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.

the cracks there would probably be caused by some kind of deformation during use, so maybe a couple of wraps of 1" fibreglass tape and epoxy resin. cutting away the damage would not prevent the damage happening again with use.
( or did somebody drive over the tip, like one of my masts)

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.

I cut 150mm off the bottom end of the top half of a North mast because I wanted a 4150mm mast (stronger than a 4 mtr mast with 150mm extension sticking out). That was 3 years ago and its had a flogging in that time. (it will probably split now I have said that though)