If you love MC Classic, I highly suggest a client called ClassiCube: it's super optimized and has a very friendly community. Some servers support custom blocks and different gamemodes. ClassiCube can also run on stuff like the Wii or Windows XP.
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where do i download this
is it on the 3ds
To answer your questions: https://www.classicube.net and yes, it is on the 3DS, although it runs very slow on more complex servers.
I should correct myself in the post. ClassiCube is NOT a minecraft classic client, but rather a rewritten version of it in C as opposed to Java.