To my knowledge there are no 3x3 macaroni bricks like you'd really like to be able to use here. There are 2x2 and 4x4 macaroni bricks:


A 3x3 plate exists, but it actually has an outer radius of 2 and a short straight segment:

There are some parts that might be useful, depending on exactly what you are trying to accomplish. There is a 3x3 round window element that could be useful if you are willing to add glass from floor to ceiling:

There is also a half-circle panel in this radius the could be useful:

3x3 wedge bricks could be an option if you can compromise on having a truly round edge:

There are also various ways you can get close to this angle using studs-not-on-top techniques. This can be especially useful if you are going for a brick or stone look. Here's one example:


That's not quite the 3x3 corner you are looking for, but the basic idea could be adapted to best suit your particular needs.
Here are some other possible ideas from existing sets. The towers in Kinddoms Joust:

The lighthouse in Lighthouse Point. This uses 2x2 macaroni bricks to build a not-quite-round 3x3 corner:

The windows on the Pet Shop achieve a good estimation of this radius using 2x2 bricks and hinge plates:

Here's the technique in detail:

Hopefully something in there is useful for inspiration. Feel free to ask for clarification via comments or ask other new questions about how to solve specific issues in your build.