We also thought of Emmeline (EMM)....

More random thoughts...

You could try alliteration (all MMM's)

You could find something with the 'other' vowel - Y! (i.e. KYM)

You could make the initials mean something numerical i.e. On a keypad, M=6, or it’s the 19th letter of the alphabet – you could make it all add up to something significant like your anniversary etc.

That way you can still tell your 2nd child that you thought long and hard about making their name not only interesting, but in a different way to their sibling

Failing that - you may need to look to the four letter options...this can be an amusing exercise if nothing else.

Oh and "Imogen" and "Isabelle" are nice "I" names