This is a great question, and one that I think you could get 100 different answers from 50 different brewers - but I'll share my $0.02! I personally think that adding all 3oz at once, and racking off after 5-7 days will provide the biggest hop "punch", and will also keep the beer tasting its freshest. Having a Pale Ale age for 3 weeks in secondary is a little long IMHO.
I have added different dry hops at different intervals before as an experiment, but they were different hops. I brewed 10 gallons and added 2oz Citra / 1oz Sorachi Ace all at once to five gallons (for 6 days), while adding the 2oz Citra for three days, then 1oz Sorachi Ace for another three days to the other five gallons (total of 6 days). I liked the first 5 gallons better, but couldn't notice a huge difference to be honest. If you were to add an ounce every two days for 6 days that might be a good schedule, but carrying the secondary on for 3 weeks on a Pale Ale is a little long. Just my thoughts though...I'm interested to see what everyone else thinks! Again, great question stoneyts...