Entry title h3 in a portfolio

i guess you are talking about the usage of the filter on line: 1156 – the other lines ( 3times that filter is set inside portfolio.php) allready do have h2 as default.
Can you show us that page it belongs too? You are definitly using the portfolio element to show those portfolios? Because there are many ways to show them even as masonry with portfolio-entries.

these filters are used on a different context (see line 698) too – maybe try:

function my_portfolio_heading_settings( array $args, $context, array $extra_args = array() ){
  if( $context == 'avia_post_grid' && is_array( $extra_args ) && in_array( 'not_one_column_template_special', $extra_args )){
    $args['heading'] = 'h2';            
  }
  return $args;
}
add_filter( 'avf_customize_heading_settings', 'my_portfolio_heading_settings', 10, 3 );

That was exactly my impression, which is why I provided the code above.

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for masonry it is not that complicated because inside the helper : class-avia-masonry.php there is only one instance of that filter. And no extra_args are needed:

function my_masonry_customize_heading_settings( array $args, $context){
  if( $context == 'avia_masonry' ){
    $args['heading'] = 'h2';
  }
  return $args;
}
add_filter( 'avf_customize_heading_settings', 'my_masonry_customize_heading_settings', 10, 2 );