You'll have your work cut out keeping a plant which wants to get very large as a small plant. Rosemary can get very big - normally to about 1.5 metres (4ft or so), but mine is taller and wider than that. You'll need to keep on chopping it back, which could mean you'll end up with more brown than green on it (as you remove the outer leaves), and I strongly suggest you keep it in a pot. Potted plants are always stunted to some extent, but let those roots loose in the garden, and it will just want to keep on growing.

It rather depends on what variety of rosemary you have. Some remain more dense and compact than others. Whether you can keep the conical shape I don't know. Again it depends on the variety. You might find it more amenable to a rounder shape. I believe you could keep it at 3 feet but it would need to be clipped fairly frequently. I have not found any problem in cutting rosemary hard back into old wood. In fact I chop some of the branches of mine right back at least once a year. I have often reduced the whole bush from 4 feet plus down to 2 feet and it regrows more thickly. It's at least 18 years old now I think (I don't keep records.)

Hopefully, I have some type of dwarf topiary variety since it's about 12" tall and trimmed into a cone. I'll try like Flora suggested.
I'm afraid in this heat to keep it in a pot since most things I"ve done this with have fried. I'm still trying to figure out how to do that with mint. I'm experimenting with a deep 5 gallon container but haven't gone through a summer with it so don't know how it will work.
Hoping to be able to keep my sage under control with heavy trimming and harvesting as well.

More than likely it is Tuscan Blue or Santa Barbara. Two of the most common rosemary varieties grown for topiary Christmas trees in the nursery industry. They grow fast and in the nursery industry this is the 1st priority.

I see no problem with you keeping it in a 3 foot tree shape as long as your willing to trim it 3-5 times a year. Our local post office has box hedged their rosemary over the last 10 years I have lived here in this part of Texas and these guys aren't pro's.

As to your mint challenge have you consider a SWC (self watering container) it will give the moisture you need but you will have the soil volume of more like a 7 to 10 gallon container. Might be easier to keep it alive over the hot part of summer. Helps here in Texas quite a bit for other plants I have container challenges with in summer.
Happy Growing David