How to add filter for wordpress plugin?

You are almost close.

add_filter( 'some_custom_filter', array( $this, 'filter_suppress_the_content' ), 10, 3 );
// the above line states that, the method `filter_suppress_the_content` should have three arguments, where you have used nothing.


public function filter_suppress_the_content() {
    return true;
}
// comparing your code, this method should have one argument

The solution:

public function setup_filters() {
  add_filter( 'some_custom_filter', array( $this, 'filter_suppress_the_content' ), 10, 1 );
}
public function filter_suppress_the_content( $false ){
    return true;
}