Is 3 phase line to line loads operating at lower voltage and slightly more current why some say 3 phase is cheaper to use?
When you connect a 208 V load from phase A to phase B, that is a single phase load.
When you connect a 208V load to all three phases, that is a three phase load.
For single phase line-to-neutral loads, current travels from source to load on the line wire, and back to the source on the neutral wire. You are requiring two wires to carry the current.
For single phase line-to-line loads where the lines are 180 degrees out of phase (120/240), current travels from source to load on the black wire, and back to the source on the red wire. You require two wires to carry the current. You might also require an additional neutral, for carrying any imbalance of the load, or for instrumentation purposes. This is two, maybe three wires required to carry the current.
For single phase line-to-line loads, where the lines are 120 degrees out of phase (120/208), current travels from source to load on perhaps the black wire, but it cannot all travel back on the red wire due to the phase timing. Some of it travels back on the neutral. All three wires are actively involved in the circuit, thus all needed. Only one phase of the three phase system is used, so you don't get any more value out of it than you would with an ordinary single phase circuit.
For three phase loads, current travels from source to load on all three line wires. It can travel back on the neutral, but doesn't need to, since it adds up to zero for a balanced system. The power transmitted is equivalent to the phase current multiplied by phase-to-neutral voltage, and added up among all three phases. This gives you 1.73 times as much power as an equal current/equal voltage single phase system, but you only need 1.33 times as much wire. You have more power utilization for each kcmil of copper.