as others note, must be proper dishwasher salt.
3 in 1 tablets are a waste as you still need to add salt (the ads lie, dishwashers can be damaged without it), and probably rinse aid too. Use them up, then my recommendation is to get to Wilkinsons and buy a set of their cheapo tablets, dishwasher salt and rinse aid, the lot will be under a fiver. Don't buy anything that is advertised, same thing, multiple of the price.
use half a tablet per wash. Only run the dishwasher when full. Don't do pre-wash or soak programmes - I run mine every other day and it still works happily like this. Scrape off food but don't pre-rinse dishes (you've bought a dog, don't bark!)
BTW dishwasher tabs are caustic and poisonous, keep out of reach of anyone likely to eat them.
read the manual and use the programme that uses the least water and electricity. This may not be the quickest one but who is in that much of a hurry?
if you have economy 7 and run the dishwasher overnight, make sure there is a smoke alarm in range. If the switch is accessible, turn off the power after use as modern dishwashers use standby power.
otherwise it is a 'magic cupboard' and one of the miracles of modern life! A full dishwasher uses less water (and energy to heat it) than washing up.