Isp config 3 does have some API available but not as extensive as Cpanel. The great thing about ispconfig3 is, it is light and not as confusing as kloxo or web min. I call it an awesome alternative for directadmin if you want a free solution.
Beware of the fact that unlike cpanel, ispconfig is not made for people who are Linux noobs. You don't have that web based point and click server management features such as compiling PHP, Apache and so on. But if you are comfortable with command line, I will say, go for it.
To best of my knowledge, don't use ispconfig for commercial hosting g service. It is great if you are using for your personal server or for couple of friends and family. But if you want people to pay for their hosting, besides plesk, there are no real cpanel alternative.
Fell free to ask why and I will try my best to explain it
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