I have a
select c.* from bills b
left join ***b.customer_type*** c on c.id = b.customer_id
You could join all three with necessary condition:
select c.*, u.*, s.* from bills b left join customers c on c.id = b.customer_id and b.customer_type = 'customers' left join users u on u.id = b.customer_id and b.customer_type = 'users' left join suppliers s on s.id = b.customer_id and b.customer_type = 'suppliers'
Then you can take the data that is relevant from the result.
However if there are similar columns in these 3 tables you might want to restructure the database to only store one type of information in one place.