going from 1 to 3 is it really that hard?

You always hear how having more children only gets harder and people are not always so pleasant when telling you this. I have a 6yo son and SO has a 6 yo daughter we just found out she is coming to live with us and also that we are expecting! I can't really wrap my head around this. I can't ecen begin to imagine the changes we are in for including financially .... can anyone tell me what its like to have 3 kids? All I'm hearing is people telling me I'm crazy bit it's not like this was planned nor can we change it so I'm hoping to find a way to transiton gracefully ... is this possible? :)