I have worked with 3 block raised beds before in a community garden. We filled the beds with almost 100% compost, so it was very light but had trouble retaining water. We did as basjoos suggests and staked rebar on the inside pf the blocks. Two years in there was no trouble with walls shifting.
That said, I now also build wall-less raised beds. However, these are not right for all situations. If you are putting beds in an existing lawn where you will keep most of the lawn, use the blocks or some other wall to help keep the grass out.
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