Well unless dev team share some info how they have done it, we can't really have accurate ideas, but it's fun to speculate anyway

. I am personally very interested in all 3 of Hybrids shown. Lawbringer and Nobushi has the potential of being my favorites in their respective factions.
First off, all 3 of them have polearms as main weapons, unlike other classes (well, Raider's axe is arguably also a polearm but not so clearly as these folks). So they will probably have considerable reach advantage over other classes. Also, while some look and be heavier, polearms are typcially easier to manipulate and may appear more agile than hand weapons due to superior leverage they provide.
We'll have to go by looks, and historical facts, but as mentioned this will likely be misleading as characters are made more for gameplay balance and "coolness factor". Raider for example, has by far the worst protection, but is still the tankiest of Vanguards, or nodachi of Kensei seem to be faster than Warden's longsword, which is not realistic at all. So these are just what "may be".
So, if we go by the looks, Lawbringer looks like it will hybrid between Heavy and Vanguard. It has the most advanced armour, in style of later periods compared to armour of other Knight characters, which leads me to believe it may be part heavy. Than comes the weapon, the halberd is one of the most versatile and powerful polearms ever devised. You can hammer, you can thrust, chop, hook, use it's back, what have you. Later period armour was highly advanced and granted great protection, and yet was much better articulated / weight distributed than older armours, and halberd can do many interesting things to trip an opponent. If they choose to portray such features of halberd, Lawbringer may have, for example, a pretty far reaching guard break / trip attack. But I doubt we'll see these that way, unfortunately so far the game seem to portray knights with usual hollywood / gaming troupe of "lumbering guys in armour without much finesse". So, I think Lawbringer will rather probably be a tanky fighter with some wide area & powerful attacks, however will most likely be slow in movement, and perhaps attack too. A definition which lends itself to a mix of Vanguard and Heavy. Although I like Warden most at heart, from what I see so far it seems it is modeled as an underwhelming character unfortunately. Therefore, this guy might be my main Knight character, and so the main overall.
Nobushi seems very lightly armoured, but highly agile. Probably will be a mix of Vanguard and Assassin. Probably she'll have a throw that will send recipient party pretty far away, at least her kicking attack gave me that impression in factions trailer. If so, this is a Vanguard trait so far. Also, what we saw of her so far, it seems she has wide sweeping attacks with Naginata, that cover both a large area and have a pretty long range, which is also a Vanguard trait. Looking at weapons, we can already say Naginata will be faster than Halberd, but with less types of attacks availabe, and with overall less striking authority. So yeah, I think this one is quite obviously a Vanguard + Assassin. If this class won't look "unnecessarily OP" as Orochi appeared so far, I may like and play Nobushi a lot actually. With that sort of wide and long sweeping strikes, she will probably be good at both clearing minions in dominion, and keeping multiple enemies at bay against hero characters.
Now the Valkrie, I am not really too sure of what will she be a mixture of. Only time we have seen her attacking in a video, she did an overhead chopping attack on a minion in single player mission with Raider, and it appeared somewhere between being slow and decently fast. Spear is the bread and butter of both ancient and medieval battlefields. It is an easy to use, deeply penetrating, deceptively fast and agile type of armament, which is also relatively cheap and easy to make. Main weapon of a raiding Viking was, more often a spear than an axe or sword, and main weapon of a Samurai was yumi (bow) and yari (straight, thrusting only spear) or naginata rather than popular culture icon katana, and in same way, a Knight's main weapons were usually a lance on horse, or a pole axe on foot, which is a shorter version of halberd. So I'm glad we finally see a spear character in this type of games. The basic spear can actually be said to be a lighter, faster, more agile weapon than naginata, as the latter has a more substantial blade, adding more weight towards tip. Though, in chopping attacks, it will not do as much damage neither as the naginata, nor as the halberd. Looking at armour, she does seem to be more armoured than most Viking characters besides the Warlord, but not that heavy. However, even if very small, she does have a shield, and so far only the Heavy classes have a shield in Conqueror and Warlord's cases. So, I am not sure what will she be mixing. Heavy & Vanguard ? Heavy & Assassin? Vanguard & Assassin? I am not quite sure, looking at her weapon and armour, both seems possible, and unlike Nobushi, we didn't see much of her moves. She does look intriguing to play with though.