The developers are planning to release S3 eventually, but it may or may not happen, and it likely will be a little while before it does.
Also, those who have S2 will receive a discount on S3 equal to what they paid for S2 (the same way that S2 is for S1 license holders). Buying S2 now will count towards your future purchase of S3 no matter when S3 comes out.
Also, with the USD ($) strong right now, it is probably your best bet to go ahead with S2 and then get S3 if and when it comes out... it might be a while and it might not happen.