We can use the lightning web component in the custom tab( in lightning experience and salesforce mobile app) with the winter 20 release.

If you have created lighting web component you know that there is a configuration file that represents the metadata values for the component. To allow lighting web component to be use in lightning app builder we need to set “isExposed” to TRUE. Adding “lightning__Tab” to target will allow to use lwc in custom lighting component tab.

<?xml version="1.0" encoding="UTF-8"?>
<LightningComponentBundle xmlns="http://soap.sforce.com/2006/04/metadata" fqn="HelloCustomer">
<apiVersion>47.0</apiVersion>
<isExposed>true</isExposed>
<targets>
<target>lightning__AppPage</target>
<target>lightning__RecordPage</target>
<target>lightning__HomePage</target>
<target>lightning__Tab</target>
</targets>
</LightningComponentBundle>
view raw HelloCustomer.xml hosted with ❤ by GitHub

Goto -> Setup -> Quick Find -> search “tab” -> click on tab

Click on “New” button
Select your lightning web component

Thanks

Arun Kumar