Hello and welcome on BN
I own Dior and Rochas and never smelled Brit but I am not sure if any on of this 3 can get you through hot days safely.
Out of 2 that I tried Dior is more mature, elegant and original. Someone can consider it feminine, someone too old.. For me its allright.. but just for special occasions.
Rochas is more casual gourmand that reminds me Axe Dark Temptation on steroids. Its versatile but can be very annoying in hot weather.
It really depends on conditions that you plan to use it. I ALWAYS fit my fragrance with clothes and would never wear Dior with t-shirt and shorts etc. So first tell us where and when do you want wear it (school, work, what kind of work (office, outside, etc), and what outfit do you use there. Also be sure to try them first (Rochas can be relatively safe blind buy if you like sweet fragrances but I can't say it about Dior).
I'd recommend you fragrances like Gucci PH II, YSL L'Homme, Bvlgari PH and Aqva, Chanel Allure Sport, Guerlain Homme, Chanel Platinum Egoiste or Versace PH that are IMHO more suitable as signature fragrance because they are very versatile.