They are three insulation types defined in SAE J1128, the North American standard for low-voltage primary cable in road vehicles. All three use cross-linked polyethylene insulation and are rated for use up to 125°C, well above ordinary PVC primary wire.
The difference is wall thickness. SXL has the thickest wall and suits routes where abrasion resistance matters most. GXL has a thin wall and is the usual general choice for engine-compartment wiring. TXL has the thinnest wall, which saves weight and space in dense harnesses where the routing is well protected. Choosing between them is a trade-off between mechanical protection and harness bulk, and the harness drawing should name the type.
APAR makes e-beam cross-linked GXL, TXL and SXL wires alongside ISO 6722, JASO and DIN types. See automotive wires and cables for the range.