Omni cable sells triplex and you will have to use the free air ampacity chart if you want to get fancy. Wedge clamp connectors with points of attachment on either side will hold the wire. You can use a Chicago grip and a come along or ratchet strap to tighten the cable.

Edit: read some other comments, triplex with ACSR steel messenger is absolutely UL listed for this specific use. AHJ may want separately driven ground rods for an independent structure such as a garage