find where the brick is behind plaster before drilling
If you are talking about supporting a bicycle then 100 year old mortar will certainly hold your bike especially with three anchors.

I would just put the anchors where I wanted them and not worry about hitting brick. However, with a running bond pattern, and measuring the brick dimensions from the outside, whether your first anchor hits brick or not you should be able to measure from there to hit brick on the two subsequent anchors.

Good luck!