The Onc and the Den will love to be fertilized fairly often through the warm months, but not every watering. The Phals are very slow-growing so I'd say no more than once a month give them some fert.
This is the danger with buying something that just says "orchid food" because you really can't generalize orchid care. There are so many - literally thousands of different kinds of orchids and you can't treat them all the same. Too much fert too often can stimulate a lot of big, floppy growth that is weak and unhealthy. The fert maker wants you to buy more, of course, so they'll tell you to use it often.
Phals also like a lot less light than Den or Onc and that's maybe one reason why they grow so much more slowly. My Dens outside get almost full sun through the pool screening. When the weather is full-on Florida warmth, they get fertilized once a week or so, but get watered lightly every day. My Vandas get fertilized almost every day in warm weather and once a week when it's cooler. (they are kept warm through the winter in my protected hoop house area) When it's sunny they get a short spray with the hose every time I walk by them - sometimes 3 times per day.
Everyone's growing conditions are different, too. How warm/cool your house is, how much light comes in at different times of year, and the relative humidity of the area you're growing your orchids all make a difference to how you must care for them. i.e. it might be much drier and warmer indoors when you have the heat on vs. if you open the windows.
When/if you move them outdoors for the summer months, you need to change up your watering and fertilizing routine to allow for weather, wind and the new conditions where the plants are. You also need to watch them during the day for things like the sun hitting the leaves - eg. Phals will burn very quickly if the sun gets on them in the middle of the day, say through a gap in the leaf canopy of a tree. A watchful eye for insect problems also is good when they go outside.
