Just to say, my 8 month old dropped the third nap a few weeks ago and we were having the same problem as you've had, also with the same A times. Very short in the morning, ending up with a too-long A time in the afternoon so my son was begging for a catnap.
I did what Stacy suggests and it worked, although the transition was bumpy and took a few weeks.
I managed to push the first A time just slightly longer (only 15 mins longer honestly) - my son wakes at 7am, first nap at 9am to 10am or 10.15am, then a little over three hours A time gets him to 1.30pm for his second nap. The additional A time pushed the nap longer, so he sleeps until 3.30pmish, which gets us to a 7pm bedtime.
The A times start to lengthen quickly from seven months, I found, but yes, we were in the same odd situation where my son really needed two and a quarter naps, or something. I couldn't push him longer in the morning, so he was having ridiculously early bed times and (oh no) early wake ups for a few weeks.
He went back and forth from 2 to 3 naps for a few weeks, with early bed times when necessary, and now we're solidly on two naps and it's far easier.
Most of the trick was to wait it out, and to push the A times slightly longer. You'll see he'll get ready to do it naturally. Oh, and also one other tip? It's a transitional time so expect it to be rough. I wish someone had told me that a few weeks ago! I didn't realise it would take three weeks to kick in for my son.
And now I've got nightwakings to deal with AGAIN - my son has had two colds in a row, sigh, so just when I think we're back on track, it all falls into a heap again. I'm just hoping I don't train him to wake up....