Elgato Thunderbolt™ 3 Dock has two ports that can be used with an HDMI display, assuming you have the correct adapter:
1) Thunderbolt 3 port - Use an USB-C to HDMI adapter
2) DisplayPort connection - Use a DisplayPort to HDMI adapter or cable
USB-C to HDMI adapters are more expensive than DisplayPort to HDMI adapters, so if you only want to use one HDMI display, you may want to consider a DisplayPort to HDMI adapter or cable.
You can connect two displays at the same time, assuming they are 4K or less.