First of all, there are no "top three spells of all time." Different witches cast different spells for different reasons. Some witches don't even cast spells.
You are new, so I would suggest checking out the General Info section of the forums and then reading the stickied threads (the ones with red thumb tacks next to them). That will give you a good starting point, and you can go from there. I would not suggest starting with spells just yet.
How many spells? Countless. Someone made up all the ones which you will find now, many people make theor own in stead of using one pre-made, and anyone can make their own. That makes the number of spells in existance limitless, as the variables are undeterminable.
A spell is not what is magic; it is a ritual to direct one's energies and intention towards the purpose for which they want a magical result. It opens up the possibility of what may be done as a spell or ritual to basically anything.