Another way is to use an extension called Lines to Arc. I find this the most elegant and quickest way. It works on Arcs as well as Circles.

So long as the edges are all equal length (i.e., not modified from circle or arc segments) it will turn the loose edges into a SU circle or arc object, which is often more useful than a Welded Curve object.

The Lines to Arc object is a real circle or arc, and therefore has, for example, a radius property, and also a centrepoint which you can find by the usual inferencing methods, by hovering over one of the vertices to display the centre.

I’m not sure if Eneroth AutoWeld does the same - I haven’t used it much, and don’t now remember - @DaveR may know.

It’s available from the SketchUcation Plugin Store at https://sketchucation.com/forums/viewtopic.php?f=323&t=21155

Yet another option is to use an extension to find the centre from two edges then redraw the circle using that as a centre point. Chris Fulmer’s Arc Centrepoint Finder will do that trick, from the Extension Warehouse https://extensions.sketchup.com/extension/ea1d5268-f0a9-4b9a-b898-3e8c823b7708/clf-arc-centerpoint-finder.

This one is useful if one or more of the edges has been modified, for example by drawing a line to somewhere on the segment, not an endpoint - perhaps a diameter from midpoint to midpoint across the circle.

You need to use the same number of segments as the original to have the circle exactly replace the edges you have.