Cracking mortar

You can rake out the Grout , clean off and reapply fresh grout and it should be fine

You haven't mentioned if it is walls or floor tiling. If its floor then the sub flooring beneath the tiles is not strong enough to fit tiles to and is moving. You have to use cement boards to strengthen the flooring before tiling.

Hi, if you are talking about tiling made on walls, just grout the tiles again using elastic grout as a "MAPEI FLEXIBLE WALLS AND FLOOR GROUT" available in many different colours.
If is tiling made on the floor, there may be a different problem... maybe the plywood under the tiles is rotten or damaged, so you will need to strip off the floor and replace the plywood before tiling again.
Let me know if you need me to come and have a quick look of your problem or to make an estimate for the job to be done.

Rake out the old grout and redo with new grout aslong as all your tiles are solid on the wall it will be fine

Ideally to do retiling, as moisture content behind the wall could be very high, the area needs to be dried and retiled.

I assume when you say mortar you mean tile grout? you can just regrout your tiling, but if it cracked shortly after , either you used the wrong stuff or you have movement in the wall which is causing the grout to crack

steve SDB Building and Maintenance

I'm assuming you mean the grout. It sounds to me that you only need to re-grout it. If it's in a wet area, shower area, you should re-grout it asap. Or yes it will damage the wall. If the grout has gone blackish, then the damage has already been done, which will mean re-tiling that area.

Appyling more mortar will not fix whatever underlying issue causes them to crack so soon. You need to reapply a new layer of mortar to the affected areas.

Cracked grout is caused usually by one of two things, incorrectly mixed grout or movement behind the tiles. I would suggest removing the grout and hiring a professional to regrout. If the same issue happens again then all of the tiles would need to be removed and the wall strengthened, e.g. Adding more noggins to the stud work.

If the mortar or grout has cracked I am surprised the tiles haven't moved or lifted. There is clearly some movement so I feel you have been lucky to get 3 years out of the installation. Unfortunately the best think in terms of longevity and safety would be to start again as sub surfaces will become more damaged

You’ll have to retile as there’s obviously movement somewhere for this to happen