i haven't tried the 'three-instruction forth' approach myself (which i had misremembered as being due to brad rodriguez rather than frank sergeant) but that paper was definitely a major factor in forming my viewpoint that forth is not really a programming language but rather an interactive environment that happens to have a programming language (a language the 'three-instruction forth' omits!)

you probably do need a fourth 'instruction' in practice: reset, or at least some kind of interrupt that returns you to the interpreter when you're in an infinite loop. usually there's a separate reset pin, though, which leaves memory intact so you can debug what went wrong — or, at worst, set up the thing so you can power-cycle it. usually that goes without saying, but sometimes hardware is physically far away these days...