When i tried to install launcher3, having original pixel launcher already installed app will not install

Did you even try to solve the issue yourself? I mean it's literally the first point in the FAQ... Reading that is not too much to ask I guess.
Basically, you'd have to uninstall the Pixel Launcher you already have, and then install Rootless Pixel Launcher. In case the Pixel launcher is part of a custom ROM you're using, you'll need root privileges to uninstall it.
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Basically, you'd have to uninstall the Pixel Launcher you already have, and then install Rootless Pixel Launcher. In case the Pixel launcher is part of a custom ROM you're using, you'll need root privileges to uninstall it.