Step 1: Read the question.
We must find the statement about lanthanoids that is NOT true. So three statements are correct and one is wrong.
Step 2: Check the conductor statement.
Lanthanoids are metals, and metals conduct heat and electricity well. So this statement is true.
Step 3: Check the paramagnetic statement.
Most $\text{Ln}^{3+}$ ions have unpaired electrons in their 4f orbitals, so they are strongly paramagnetic. So this is true for the series in general.
Step 4: Check the coordination number statement.
Lanthanoid ions are large, so they can fit many ligands around them and often show coordination numbers above 6, like 7, 8, or 9. So this is true.
Step 5: Check the non-radioactive statement.
Most lanthanoids are stable, but one member, promethium (Pm), is radioactive. So saying all of them are non-radioactive is false.
Step 6: Pick the false statement.
Because promethium is radioactive, the wrong statement is that they are all non-radioactive, which is option 3.
\[ \boxed{\text{These all are non-radioactive}} \]