If I understand your question you're wanting to know if you can hold the ferment at its recommended temp range for the first few days
only, then afterward let it return to ambient temperature.
Yes, holding it within its recommended range is important, especially during lag and growth phrases, but temperature continues to affect yeast throughout fermentation and conditioning. At the end of fermentation, temperature can affect the yeasts' ability to clean up off flavors.
In my (so far limited) experience, the most important thing is to avoid temperature swings. They're easily avoided by dropping the fermenter in a bin or trash can filled with water to wort-level and storing it in a room that doesn't have big swings in day and nighttime temperature. The water serves as an insulator. I've got best results from using a rather large plastic bin that holds about 10 gallons of water plus the fermenter. More water equals more insulation.
Depending on your needs, you can warm the fermenter by dropping a submersible temp-adjustable aquarium heater in the bin (not in the wort!). I got one from the fish store for about $10. If you need to cool it, fill the bin with a few inches of water, drape a big towel over the fermenter with the ends hanging in the water and point a fan at it. As the water evaporates it will wick heat away from the fermenter. If you need to cool it more, add ice (to the bin, not the wort
).
But the bigger question is, if you can maintain recommended temp for 3 days why not 30? Ultimately you're trading convenience for a great batch of beer.