How to use diff3 with ed

Question

How to use diff3 together with ed? My attempt below does not match diff3 --merge output, so I must be doing something wrong.

Context: I've ported OpenBSD's diff3prog.c utility to be used in BusyBox, which has diff and ed.

cat <<- EOF > parent.txt
1
2
3
EOF

cat <<- EOF > ours.txt
1
2
3
EOF

cat <<- EOF > theirs.txt
1
2
3
4
EOF

f1=ours.txt
f2=parent.txt
f3=theirs.txt

diff3 --merge $f1 $f2 $f3 > merged_good.txt

diff3 -E $f1 $f2 $f3 | ed $f1
#prints:
#6
#?

mv $f1 merged_bad.txt

cat merged_good.txt
#prints:
#1
#2
#3
#4

cat merged_bad.txt
#prints:
#1
#2
#3

Answer 1

(originally I posted this in the comment above. no-one added any further context, but the problem is solved, so I'm adding it as the answer as well)

It seems that diff3 output misses w ed command. So to get expected output, one should (diff3 -E $f1 $f2 $f3; echo w) | ed $f1.