I treat all all three fairly and with the same level of love. I treat them equally, but differently - they are different ages, and different people.
In terms of Christmas, birthday, Easter egg, 'event' presents, etc - they get the same amount of money spent on them. In terms of ad hoc treats - varies on where we are, who we are with, etc. but those are smaller values anyway.
My sister's boy - my nephew - is the youngest. I only see him when his mum is around, as he lives a distance away, isn't very old. I adore him and love to see him, wish we saw him much more often.
DH's brother's girls - my nieces (and yes, they are MY nieces as well as DH''s; thats how it works in our family) - are a little older, one almost a teen. I love them equally as much, see them a similar amount generally and they're a similar distance away. But as they are older, and their parents have jobs without so many holidays, and they've less 'childcare' options we have them stay for a few days once or twice a year in the holidays, and me and teen DD help look after them. We have a week of day trips and activities - so I guess in this way they get more pet on them, but thats just circumstances, and day trips makes my life easier too ;) I'd be happy to look after my nephew an equal amount of time, but he's not yet at that stage, and besides there isn't the need either - my sister works PT and my parents (plus her MIL) like to have him for the day as well.
OP, in your situation I would be treating them all in the same way, so long as they wanted to. Or at least I hope I would, if it was possible to do so. I don't have favourites in my nieces/nephew - they are all lovely in their own ways, and I'd happily spend time with all three any time.