if we decide to go with a 3rd (is that crazy?)
DD is our 4th, I'm 38. Our others are 10, 9 and 5. Would have had our fourth 3 years ago, but took a while to conceive and had two miscarriages. My first 3 were conceived in first month of trying so it was very unexpected to take a long time to have #4. So I definitely give the vote for TTC sooner, rather than later.
it's been very different for us 2nd time around
Absolutely. Your third will be different again.
For what it's worth, by the time my DC1 was 8 months old (and walking unaided), I was already 3 months pregnant with DC2.
With hindsight I wonder how much our decision to have two children in quick succession was a lot swayed by my desire to not go back to teaching after maternity leave. I went back full time for 4 months between pregnancies, 3 months after DC2's maternity leave and then gave up work to be a SAHM. I believe it was the thought of returning to work that focused my mind on conceiving again.