Your 3 monitors would require 3 ports (1 port per monitor). Are your Asus monitors exclusively HDMI inputs only (no other ports like DisplayPort or DVI)? If they are only HDMIs and you want a video card that has 3x HDMI ports, some Gigabyte GPUs offer such number of HDMI output ports and are available in the market today:
GIGABYTE Nvidia GeForce GTX 1050 Ti models has
3x HDMI, 1x DP, and 1x DVI-D:
Gigabyte GTX 1050 Ti G1 GAMING 4G (https://www.gigabyte.com/Graphics-Card/GV-N105TG1-GAMING-4GD#kf)
Gigabyte GTX 1050 Ti WINDFORCE OC 4G (https://www.gigabyte.com/Graphics-Card/GV-N105TWF2OC-4GD#kf)
Gigabyte GTX 1050 Ti WINDFORCE 4G (https://www.gigabyte.com/Graphics-Card/GV-N105TWF2-4GD#kf)
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GIGABYTE AMD Radeon RX Vega 64 and RX Vega 56 models has
3x HDMI, and 3x DP:
Gigabyte RX Vega 64 GAMING OC 8G (https://www.gigabyte.com/Graphics-Card/GV-RXVEGA64GAMING-OC-8GD#kf)
Gigabyte RX Vega 56 GAMING OC 8G (https://www.gigabyte.com/Graphics-Card/GV-RXVEGA56GAMING-OC-8GD#kf)
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However, if your Asus monitors does in fact have another input port (such as DisplayPort or DVI-D), then, you can pick almost any GPUs having at least 3 output ports (mix of DisplayPort, HDMI, and DVI-D) and just use an adapter to connect the cables from your monitor to such GPU.