The PG348Q and X34 use the same panel which is 75hz native with a chance at 100hz if youre lucky, and thats assuming you also won the lottery in terrms of backlight bleed, flickering, and bad scan lines, both of them have had serious quality issues which is why the second hand market is so saturated with them
the Omen X35 uses the same VA panel as the Asus Z35p which is known to have some nasty ghosting issues and weird backlight glow, worst of all is that Acer and Asus arent being great when it comes to honoring warranty
the Alienware is 100hz native with an out of the box guaranteed 120hz OC, its IPS with LG's newest gen panels and has about as little light bleed as possible on an IPS panel, it has a better stand than the others and has a pixel perfect guarantee for even a single pixel, TFT centrals review says its the best ultrawide gaming monitor currently on the market