Yes compound errors are a concern, but sometimes two sightings won't do, say when there are ten hills in the way :-) Three poles were good enough for IKB, so they're good enough for me!

There are variations on the theme that leave the first pole in place and alternate the second two as long as you can. This does reduce compound errors somewhat.

Another way is to buy a feckin GPS! Take areading at all three sites and plot them on some graph paper. If you can draw a straight line through them then you're not doing too badly.

To be sure you need to apply lots of line-bending (and mind bending) maths to the points to get lines on the Earth's surface etc, but over very short distances this shouldn't be a major concern. Many people draw a line on a map with a permanent marker and say that sites beneath the scaled 400m wide line are in an alignment, so a little calc error isn't that important.