Yes its normal,deep down you are your Sun sign (Cancer) but you don't show it to everyone your Cancer side,cause ascendant shows how do you act around others and your asc. ruler in Leo would bring some Leo traits and maybe looks ^^
I also have Sagittarius ascendant with Jupiter in Leo ^^
To answer this, you must look at the Aspects and not just what Sign each Planet is in. And this is why the Degrees are so important. The Degrees will pinpoint just how strong an Aspect is.
The Conjunction, by Sign at least, of Jupiter, Venus, and Mars in Leo is a very strong Aspect.
Look at what your Sun and Moon are Aspecting. Sun in Cancer, and Moon in Pisces, are Trined by Sign, BUT, are they Trined by Degree? If these Luminaries are weak, (i.e. unaspected by Degree), then they are not playing such a huge role in the Chart, other than the fact that the Sun is always our goal, or task, in life... with the Moon representing our Subconscious mind (the instinctual nature and habit patterns ingrained in us from past-life experience and carried over into this life).
Your Sun in Cancer is, by Sign, Semi-Sextiling all that Leo... but... is it by Degree?
Your Moon in Pisces is, by Sign, Square your Ascendant and Inconjuncting all that Leo... but... is it by Degree?
Jupiter, Venus, and Mars in Leo are, by Sign, Trine your Ascendant... but... are they by Degree?
Your Sagittarius Ascendant is what makes you want to be free.
So, as you can see, you've got some more consideration to give as to what the strongest Aspects-by-Degree are in your Chart. If that turns out to be a Stellium, (3 or more Planets conjunct in one Sign-- in your case that Jupiter, Venus, and Mars in Leo), then that will be the focal-point (the 'dominant' point) of your Chart.
It appears, with the little information you have provided, that the Cancer Sun and Pisces Moon are providing the 'coolant' to the Leo engine.
Also, our Charts Progress as we move thru life. I don't know how old you are, but, by the description you have given of yourself, I wouldn't be surprised if your Cancer Sun has Progressed into Leo, thus coming to conjunct your Jupiter, Venus, and Mars. This will exaggerate even more the Leo expression of those Planets. The Sun, (Ruler of Leo, the 5th House, and the 'I Will!' in the Natural Chart), is far more comfortable in its Fixed Fire Sign than in a Cardinal Water Sign, (Cancer). In short, if he's conjuncting, by Progression, your Planets in Leo, he is commanding them to 'Shine! Shine! Shine!'.
EDIT: 'WELL I'M 20 AND MY BIRTH DAY IS JULY 2ND SO I DON'T KNOW IF IT WOULD PROGRESS TO LEO BY NOW'.
You have to look at what Degree your Natal Sun is in. Every Degree represents One Year. So, if your Natal Sun is, say, 15-Degrees Cancer, then, at 15-years-old, it would have Progressed to 0-Degrees Leo. Add five more years, (because you're twenty-years-old now), and the Sun would be at 5-Degrees Leo. See?
What is your Birth information? I will look it up for you and post your Natal & Progressed Charts. I need your Birthdate (Month, Day, Year). Your Birth Time (hour and minute, also a.m. or p.m.). And I need the Location of Birth (city, state, country).