convert DirectTV 3 cables to connect to Smart TV

Most TVs still have the A/V ports. But with a smart TV, you should use HDMI anyway otherwise you will get a poor picture.

What model is the DirecTV box?

This will work for you -*note you will Need to also have a HDMI cable on hand to connect to your new TV 

Amazon.com: RCA to HDMI,AV to HDMI Converter,ABLEWE 1080P Mini RCA Composite CVBS Video Audio Converter Adapter Supporting PAL/NTSC for TV/PC/ PS3/ STB/Xbox VHS/VCR/Blue-Ray DVD Players: Electronics

They shouldn't need to covert A/V to HDMI unless either they have a very outdated MPEG-2/SD-only box that needs to be replaced anyway to continue using the service, or the HDMI port is damaged meaning they need a replacement.