I've had a lot of difficulty with 3s. Personally, I think the conflict for me comes from one root - the Type 3's need for validation. I'm not trying to generalize 3s, nor place the blame on 3s rather than 8s, but I am explaining from my perspective.
In my 8-3 interactions, the biggest problem I have always had is never feeling safe around them. The closer they move towards to me, the more I want to control them so I can keep a safe distance. But I have a very hard time determining what their true motivations and feelings are as compared to any other type, whose are generally pretty cut-clear to me. I've had 3s get close to me, telling me that they care about me and such, and they are very good at making me believe this; but I detect a ton of inconsistencies, because of the need for validation. For example, one 3 would tell me I was her best friend, talk about how close we were, and she actually made me feel special. But it took me until a lot later to see that she would just call everyone was her best friend and she would comment about how close she was with so-and-so, and thereafter I realized that I was just another person validating her self-worth.
I need truth, honesty, and integrity, and if you are going to warp the truth for as stupid a reason as petty validation, we are going to have problems. The compromise I suggest here is to be truthful and authentic, and I'll give you respect and validation for that. The problem with this compromise is that having only one person to validate the 3 is just 'not enough,' as it has seemed.
I think another place of friction in the 3-8 pairing is the 8's need for control. Both 3s and 8s have a natural gravitas, but they attract weight from different sources. 8s attract people who can take part in the 8's power, while 3s attract people who love 3s for what they project. I believe this applies mainly to the so-8s (mostly speaking for myself here), but I hate losing control with or over a social group. The 3 can leech my control over the group by controlling through manipulation rather through force. It creates a source of anger for me - they can control a group, but it's just by making people like you rather than personal merit.
Honestly, that's just unfair, and I have had my credit undermined by 3s who have more simply because they're more well-liked. It drives me crazy. It becomes a battle for power, but not the same one we're looking for individually. Think of the power as a regular Venn Diagram - 3 and 8 have their own spheres of control, for different topics, but the overlap in the middle is what they battle over. But they're so different that it's very difficult to find a compromising ground to work with. 3 says, "I want you to like me," while 8 says, "I want to feel safe around you." 3 becomes intrusive as he desires to win the approval of the 8, which triggers defensive reactions from 8. They're pushing against each other but they don't understand why.
Here's a metaphor for the last sentence: The 3 walks up to the 8 for a hug. The 8 hasn't decided yet, though, if he wants a hug yet or not from this stranger. So the 8 extends a hand out saying, "woah, wait a second here." But if the 3 keeps moving for the hug, the 8 will end up holding the head of the 3 with an extended arm, while the 3 is pushing against the 8's body for the hug. They don't get away from each other, but they're still pushing against each other and they're very well aware of it. However, neither is generally willing to lay down his hands. If the 3 lays down his hands, he does not get the hug he desires, and may harbor resentment for it. If the 8 lays down his hands, he risks being crushed by the 3's hug, and if he desires to end the hug or escape from it, he may have to thrash around to get out.
I regress back to the concept of 'truth,' as I think it is the main source of conflict. I draw the line very darkly when it comes to violation of integrity. 3 will receive negative or no affirmation from me if they are not being authentic or if I feel they are not being authentic. 3 will continue to push for affirmation, sometimes without noting my boundaries. When the 3 gets rejected in favor of my boundaries, they harbor resentment for me - therefore I have an unnecessary enemy who I have to be even more careful around.
Having even said all this about 3s, my best friend is actually a 3w2 sx/so. It angered me a lot in high school how he was so popular and people would rather gravitate toward him. However, I think the compromise comes when the both sides drop their guard - that is, the 8 becomes vulnerable, while the 3 becomes authentic (masks acting as a guard). In order to achieve this, from my perspective, the 3 needs to realize that he can remain authentic and receive the validation of worth (and will get it from me if so), so the 8 can let his guard down and feel safe. If they're not willing to or don't realize it, just don't step in my 'territory,' whether that be personal boundaries or 'leeching' my followers so you can gain your own (that will righteously piss me off).
In my experience, I think this couple actually has a huge amount of potential for mutual growth - it can just be a really rocky road to get to that point, if it ever gets there at all.