Reality is there's not a definitive answer to your question - "easy" or "difficult" are contextual and perspective based.
This is subjective, but in my experience I'd say about half the candidates we find fall out of consideration as fakes/flakes/no-shows. That adds to the logistical difficulty of finding a suitable sex partner for an MFM (or even MF) encounter.
Of the remaining half, probably 50% of those are overtly homophobic, and/or have other issues, like performance anxiety - they can't get hard, or stay hard in the presence of another male. Some fraction of those that do manage an erection are too self-conscious or anxiety-ridden to cum.
That leaves about a quarter of the men we find who can both perform, and experience orgasm in an encounter where I'm present - that doesn't qualify present and participating. Within that nominal 25%, some fraction are "equal opportunity," in that they don't cringe or freak at accidental male-male contact, which is truthfully pretty much inevitable in an MFM encounter. Some contingent in this subset are overtly bisexual, and welcome/enjoy male-male contact, although the focus is still typically on the woman. There's a contingent that are what I'd categorize as "bi curious," and will reluctantly, tentatively explore opportunity during an MFM threesome - acts like the wonderful "double-barrel blow job," fuck-licking, inserting my cock into my wife, or me inserting his cock into my wife, that sort of thing.
Communication and expectation setting before the encounter are helpful - establishing limits, boundaries, things that are of interest will sometimes help make things less difficult.
The situation / context also factor in. In spontaneous pick-up situations, if I tell the target something like, "My wife thinks you're hot... would you like to help me fuck her?" will get something like a 50% or higher rejection rate - the guy usually thinks he's being "gay baited," and is usually homophobic. Making a minor adjustment, as in having KK make the offer cuts the rejection rate to less than 25% or so.
Using my current situation as a working example, KK's boyfriend, Andy isn't overtly bisexual. Neither am I. We both consider ourselves "contextually bi," in that we don't seek out bi male encounters, but we are both willing to enjoy that in the context of pleasuring KK. Again, labeling in the MFM, and MMF encounter context, we will enjoy the moment. That's important for certain pleasures, like fuck-licking, DP/DVP.
