Why are Truth, Conjunction, and Implication called "Negative" fragments of IPL (intuitionisti logic Proposition Logic)? 
Someone answered that negative means we are ""Using"" them .

But the point is for all of these there is an Introduction rule too. So why call them negative?
I don't know whether it's computer science or Mathematical question, but still this doubt persists. 
 A: Coming at this from the perspective of Martin-Löf Type Theory, I'd say it's because they are implemented by negative types.  What this means in a more general setting is that their elimination rule(s) uniquely determine(s) their introduction rule(s), as opposed to a positive type (or operator, or connective), whose introduction rule(s) uniquely determine(s) its elimination rule(s).
To explain concretely for your particular case:


*

*$\land$ is a negative connective because its elimination rules $\forall P, \forall Q, P \land Q \to P$ and $\forall P, \forall Q, P \land Q \to Q$ uniquely determine its introduction rule $\forall P, \forall Q, P \to (Q \to P \land Q)$.

*$\lor$ is a positive connective because its introduction rules $\forall P, \forall Q, P \to P \lor Q$ and $\forall P, \forall Q, P \to P \lor Q$ uniquely determine its elimination rule $\forall P, \forall Q, \forall R, (P \to R) \to ((Q \to R) \to (P \lor Q \to R))$.


Similar things apply to  $\top$, $\bot$, and $\to$.  That being said, $\top$ and $\land$ can both be considered positive as well, and they bifurcate when one considers linear logic.
A: Each of implication, conjunction, and truth is a "right adjoint" in categorial semantics. This means that to each of these operations is associated a category in which the operation is a final object. 
Truth is final in the base category.
Conjunction of A and B is final among propositions implying both A and B separately.
Logical implication from C to D is final among propositions Z such that the conjunction of Z and C implies D.
Visually, final objects look like sinks in a category, hence they're "negative."
Positive connctives are left adjoints, hence initial, hence they look like sources in a category.
Generally speaking, the negative component is more expressive than the positive component because conjunction has a right adjoint, logical implication, while disjunction only has a left adjoint if the logic satisfies excluded middle (correct me if I'm wrong, please).
