Every fire sign for sure share some similarities as being straight, active, energetic and passionate, but there are substantial differences between them.
Aries is the primordial fire, the big bang, the abrupt generating energy needed to start anything new. Aries' fire doesn't compromise and doesn't think twice. Action is the priority.
There's no constancy in Aries' fire, but the initial outburst of energy is the strongest of the zodiac. Such kind of fire is necessary for starting new cycles in society and individual people's lifes.
Innovations, revolutions and individual's new beginnings often begin with a destructive energy that makes space for building something new from scratch. Pluto/Scorpio energy in this regard often is synergetic with Aries'.
Leo's fire is a more mature kind of fire, a constant source of warmth and optimism as the Sun is, which is Leo's planet, or as the heart unrelenting beat.
Leo's fire shines without hiding and with generosity, showing its energy and light to the world, well aware of his self-worth, sometimes to the point of showing off.
Leo can channels the fire also through creativity and pro-creativity, and by sharing the warmth of the heart by supporting others.
Sagittarius' fire is even more refined. While Sagittarius' fire can also express itself with enthusiasm, optimism and action (sport and traveling), it also conveys fire in the inner world as a quest for knowledge and for going beyond the borders of the ego through spiritual journeys.
Sagittarius can transform itself in the quest for inner truth. The exploration loving Sagittarius turns the journey to the inner world and can recognize the ego for what it is, as an impostor. The very fire and passion for truth can then burn the ego leaving space to a higher form of spiritual identity.
All 3 kinds of fires are needed in different stages of our life.