Sounds like he is having a longer night sleep than most babies - I've read 10-12 hours is normal overnight. So he may be making up for all the sleep then by only having 2 day sleeps. I would think at this stage it would be too hard introducing another day sleep, especially if you are following his tired signs as most babies tend to drop the third day sleep around 8 months or so, so you would be getting rid of it again soon. As she gets a bit older she probably won't be so tired at the end of the day, or you could bring forward bedtime 30 mins or so and see what happens.

I wouldn't count that 6-8am sleep as a 'day sleep'. I consider day sleeps as sleeps that occur after they have been up and had some play time.