If your baby is around 8 or 9 months old, often takes a long time to fall asleep for their third nap, or it seems to be messing with their bedtime routine you might want to try cutting out the third nap and bringing bedtime earlier.
If they are having good naps at around 9am and 12.30/1pm then they should be able to last until bedtime especially if their bedtime is flexible depending on the quality and quantity of their daytime sleeps. I would recommend setting their bedtime at 3 hours from the time that they woke up from their second nap. This may mean that on some days bedtime may be as early as 5pm, and other days it may be 6.30pm.
If you find that there is a day that they just aren’t going to be able to last, try giving them a half hour catnap in the pram or car at about 3.30. Because they will be moving, this nap won’t be restorative but it will be enough to tide them over until bedtime.