simplifying RTD (resistance temperature detector) units
That is not &quot;dissipation constant&quot;, it is the temperature coefficient of resistance.
What they are saying is that if you have a 100 ohm RTD it will be 100 ohms at 0°C and 138.5 Ohms at 100°C (using the most common DIN 43760 standard with \$\alpha\$= 0.003850).
So the average temperature change per degree C from 0°C to 100°C is 0.385 ohms, and that is 0.00385\$\Omega/\Omega/°C\$ (base resistance taken at 0°C).
Similarly, a 1000\$\Omega\$ RTD with \$\alpha\$=0.00385 will change from 1000 ohms at 0°C to 1385.0 ohms at 100°C.
RTDs are nonlinear so different temperatures and ranges (other than the commonly used 0°C/100°C) will result in slightly different numbers for the same RTD.
The three different numbers you have represent slightly different platinum alloys or made with different construction (thin film vs. wirewound on pure alumina etc).
You can use \$°C^{-1}\$ as the unit, the dimensionless \$\Omega/\Omega\$ are to remind you of what it represents.