Often a spline is used in a timber frame to reduce the amount of wood that is taken from a post or beam, this particular connection detail has two knee braces flanking the spline.
Often a spline is used in a timber frame to reduce the amount of wood that is taken from a post or beam, this particular connection detail has two knee braces flanking the spline.
Is there a measured detail for the corresponding mortises for this knee brace and spline?
Take a look at the image at https://timberframehq.com/36-knee-brace-piece-drawings/ it has some additional dimensions that will help you out.
Is there any option for three beams connecting to a post?
You can use the spline joint similar to this one but the timber coming in from the side use a top spline, not the bottom one as shown. See the detail at https://timberframehq.com/four-beams-into-a-post/
Can the spline and corresponding notch(s) be full depth of the plate, in other words, saw a notch from the top of the post down equal to the plate depth, and saw the corresponding notch in the plate from edge to edge (top to bottom in this case)
I would not recommend it as you end up taking too much meat out of the timbers. The best way to cut them is with a chain mortiser.