The standard model(SM) of particle physics is an experimentally tested successful theory of the modern physics era. However there exist phenomena which the SM does not describe. Among others, the neutrino mass hierarchy which has been observed by experiments like Super-Kamiokande has no explanation in SM. This requires an extension or perhaps new physics beyond the SM. One of the extensions for the standard model, type-III seesaw mechanism introduces a fermionic triplet, which can explain why the neutrino masses are small and why they indeed have the mass hierarchy. The fermionic triplet in type-III seesaw mechanism decays to the multileptonic final state via electroweak interactions. The multileptonic final decay has an advantage of being rare, relative to other standard model backgrounds. In my thesis, I have undertaken a search for type-III seesaw signal in events with 3 or more leptons (e, mu, and tau). The data on which this search has been performed corresponds to 35.9 \fbinv of integrated luminosity in pp collisions at $\sqrt{s} = 13$ \TeV collected by the CMS experiment at the LHC. Since the signal populates channels with at least three leptons and diverse kinematic properties, the data is binned in exclusive channels. The primary selection is based on the number of leptons and the invariant mass of opposite-sign dilepton systems which helps discriminate the signal against the Standard Model background. The final optimization for the type-III seesaw signal is based on the sum of leptonic transverse momenta and missing transverse energy. Control samples in data are used to check the robustness of background evaluation techniques and to minimize the reliance on simulation. The observations are consistent with expectations from Standard Model processes. No sign of type-III seesaw fermion production is observed. The results are used to exclude heavy fermions of the type-III seesaw model with masses below 840 \GeV.
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