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We present optical follow-up observation results of three binary black holemerger (BBH) events, GW190408_181802, GW190412, and GW190503_185404, which weredetected by the Advanced LIGO and Virgo gravitational wave (GW) detectors.Electromagnetic (EM) counterparts are generally not expected for BBH mergerevents. However, some theoretical models suggest that EM counterparts of BBHcan possibly arise in special environments, prompting motivation to search forEM counterparts for such events. We observed high-credibility regions of thesky for the three BBH merger events with telescopes of the Gravitational-waveEM Counterpart Korean Observatory (GECKO), including the KMTNet. Ourobservation started as soon as 100 minutes after the GW event alerts andcovered 29 - 63 deg$^2$ for each event with a depth of $\sim$ 22.5 mag in$R$-band within hours of observation. No plausible EM counterparts were foundfor these events, but from no detection in the GW190503_185404 event, for whichwe covered 69% credibility region, we place the BBH merger EM counterpartsignal to be $M_{g}$ > -18.0 AB mag within about 1 day of the GW event. Thecomparison of our detection limits with light curves of several kilonova modelssuggests that a kilonova event could have been identified within hours from GWalert with GECKO observations if the compact merger happened at < 400 Mpc andthe localization accuracy was of order of 50 deg$^2$. Our result gives a greatpromise for the GECKO facilities to find EM counterparts within a few hoursfrom GW detection in future GW observation runs.

GECKO Optical Follow-up Observation of Three Binary Black Hole Merger Events: GW190408_181802, GW190412, and GW190503_185404