Override the FLBuilderGalleryGrid resize method to set a custom number of columns or adjust the 480px breakpoint.

fl-gallery-grid-override.js

( function( $ ) {
	
	$( function() {
	    
		FLBuilderGalleryGrid.prototype.resize = function()
		{
			var winWidth    = $(window).width(),
				wrap        = $(this.wrapSelector),
				wrapWidth   = wrap.width(),
				numCols     = winWidth > 480 ? 3 : 1,
				items       = wrap.find(this.itemSelector),
				itemWidth   = wrapWidth/numCols,
				itemHeight  = itemWidth * this.itemHeight;
				
			// Browser bug fix. One column images are streched otherwise.
			if ( 1 === numCols ) {
				itemWidth -= 0.5;
			}
			
			// Set the item width and height.
			items.css({
				'float'  : 'left',
				'height' : itemHeight + 'px',
				'width'  : itemWidth + 'px'
			});
		};
	} );
} )( jQuery );

Note to ditch the ; on line 13 :)