Hi Lin and Alan
Great to see you coming to Spain, I am sure you and your dogs will have a wonderful time. I have had dogs most of my time in Spain. I have always used a dog seat belt with my dog, which was a harness whihc had a belt attached and that clipped in like a seat belt, to make sure in case of a an accident the dog didin't come flying through the front seats. However having three I assume you will have them in a large boot. If the dog is in a the boot you should have a saftey barrier in place between the boot and the second row of seats. This is sufficient. They do not need to be in "cages". If they are travelling on the back seats, they should be "belted in". The law stipulates that dog must not be loose in the car and certainly should not be able to move into the driver's area. If you are caught with a dog loose in the car you could be fined up to 100€, it may be more now. If the dog is found in the front seat area you could be fined over 200€ and even charged with negligent driving. If they are secure in the boot with a protection barrier installed that is fine, it is a contained environment and safe for the driver and other passengers.
It is not necessary that wear a muzzle when travelling, nor infact is it necessary that they wear a muzzle when walking them through the street, unless you know you dog is going to do something it shouldn't or unless they are considered "dangerous breeds" such as pitbulls, rotweilers etc In whihc case they must be muzzled at all times and you must have an insurance policy to cover any incidents.
This is my understanding of the law and I have been stopped before and never fined. My dogs have always been Golden Retrievers, which do not need to wear a muzzle.
With ticks and fleas, they are more common in the summer period, I used parasite collars , the best one I found was "Scalibur" which costs 15 euros a collar, I've used them all, but this gave the best results. They last for approx. 6 months and worked very well. When the Summer comes along in June/July and August I always made sure I started around mid June with a new collar and gave him a "Frontline" parasite treatment that comes in a monodose format. You put the liquid on the skin between the shoulders. Very good and effective, I did this in July and August in combination with the collar. (it will depend on the climate and the temperature, I am in Valencia, so in Malaga you may need to start a little ealier) The rest of the year, just the collar. It is also important to get the yearly vaccine for Leishmaniosos, the disease passed on by mosquitos. This can be fatal and I have seen two dogs die from it, really horrible. This is very important as there are millions mosquitos in Spain and possibilities are fairly high of being bitten by one. Go to your local vet as soon as you get settled in and programme the vaccine. It's not expensive and is done once a year just before the summer.
Hope this helps and welcome to Eye on Spain!! :)
Ian