I can confirm that your chances of getting a 3* ninja on single pulls are relatively just as great. I've done singles for my last 80 or so pulls on both servers I play, and have gotten 2* and 3* ninjas within each 10 scrolls drawn. I've also pulled my rares on singles as well, often times well before the next even ten draw.

Meaning, instead of waiting to have a full 10 seals and doing my 6th ten draw, I pull my 51st & 52nd seals and so on individually and have gotten my rares. It is luck based at this point of course, as is the probability of getting a 3* by this means. Your mileage may vary.

I've been doing this on summons as well and it seems to be working much the same way. Within each ten scrolls I'll get a purple summon which for me results in summon tokens.