Yes the snails' waste (they produce a lot!!) could be adding to the ammonia but what are they doing in there? Hitch-hikers on the plants? Not a good idea to have livestock in high ammonia levels regardless of whether they were invited or not. Also they are confusing your test results.
You could remove the snails and see what happens in a day or 2. The fact that you have some nitrite means some ammonia is being converted. The plants will be using up some of the ammonia too so the fact that it is still 3 ppm indicates it would have been much higher without the planting.
Because in effect you have been adding more ammonia (in the guise of snail waste), you may find that the nitrite level goes beyond what is healthy during a cycle and you might have to consider doing diluted testing with R.O water or D.I. water to find out just how much you have as most test kits don't go much higher than 5 ppm.
Might be worth cutting your losses by getting rid of the snails and starting again now. The fact that you have some ammonia munchers already will give you a kick-start if you do decide to start over.
