Add Contact as Related List to Opportunity, Contract and Case

Add custom object look up field rom Contact object:
 
Step 1: Go to Setup. Click on Object Manager. Search and choose 'Contact'
Step 2: Choose Fields & Relationship. Click on New button. And choose the following criteria below:
Data type: Lookup Relationship
Related to: (choose the object you want to add contact as related list - Opportunity, Case and Contract)
Provide field label and setting. Add the field in your page layout
Step 3: Save changes

Add Contact as Related list to your object page layout:
Step 1: Go to Setup. Click on Object Manager. Search and select on the object you want to add contact as related list
Step 2: Choose Page layout. Click on Edit along the page layout. Choose "Related list" from the palette then drag Contact into your Object page layout related list section.
Step 3: Save changes