You don't need to test calcium, alkalinity, and magnesium in a softie-only tank as long as you are doing at least 10% weekly waterchanges or 20% bi-weekly (is even better) since the waterchanges will keep the parameters within reason. I would suggest you use a salt that has reasonable calcium and alkalinity levels though, like H2Ocean or Reef Crystals.
If you start to buy LPS hard corals like Frogspawns, Hammers, Plates, Duncans, Candy Canes, then you will probably need to start testing and dosing cal, alk, Mg because these corals use these minerals to build their skeletons and will deplete the water. Soft corals do not use these minerals at nearly the same rate, so regular waterchanges will usually keep up.
You can always take a sample to your LFS once a month or so and get them to test cal, alk, and Mg for you. This will be cheaper than buying the kits yourself, plus you won't be testing as often on a softie-only tank as you would on a hard coral tank, so the kits will expire before you use them up.
I would definitely suggest you get better tests kits for nitrate and phosphate though. Elos is my preference, but Salifert is good too.
How long has the tank been set up? How long have the corals been in the tank? Do you have any algae in the tank? How long do the lights run for? What is the temperature in the morning before the lights come on? How about at the end of the day just as the lights go out? What do you keep salinity at? How are you testing salinity? Are you adding/using anything else on the tank like carbon, phosphate media, other additives?
Could you post a pic of your tank and sump? A picture tells a thousand words.
