functions.php

add_action( 'woocommerce_before_cart', 'apply_matched_coupons' );

function apply_matched_coupons() {
    global $woocommerce;

    $coupon_code5 = '5percent';
    $coupon_code10 = '10percent';
    $coupon_code55 = '15percent';

    if ( $woocommerce->cart->has_discount( $coupon_code ) ) return;

        if ( $woocommerce->cart->cart_contents_total >= 50 && $woocommerce->cart->cart_contents_total < 100 && $woocommerce->cart->cart_contents_total != 100 ) {

            $woocommerce->cart->add_discount( $coupon_code5 );

        } elseif ($woocommerce->cart->cart_contents_total >= 100 && $woocommerce->cart->cart_contents_total < 150 && $woocommerce->cart->cart_contents_total != 150 ) {

            $woocommerce->cart->add_discount( $coupon_code10 );

        } else {

            $woocommerce->cart->add_discount( $coupon_code15 );
        }

}