That is not true, Both entities you are wanting to fillet have to be coplanar. So if filleting an arc and line, both end points of line and both endpoints of arc and it's center point all have to be coplanar.

This dictionary has this meaning. I do not think this is correct.

https://dictionary.cambridge.org/dictionary/english/coplanar

One line is on Plane1 other is on Plane3 both are not on the

**coplanar**but fillet is possible.

You cannot do it with fillet, but can do it using construction geometry and arc. Create a construction circle on same plane as the arc and trim the arc to that circle. Create a plane in the 3D sketch using the line as first reference and the trimmed end point of the arc and second reference. You can now create arc on this plane in 3D sketch.

You already got that answer