Yes. I believe that if you buy either conquest or birthright, you get the other two for 50% off. I read that the Special Edition with all 3 sold out in just a few hours - so I was super lucky to get that. I wouldn't buy the special edition second-hand - people are selling it for ridiculous prices online but buying the other two as DLC amounts to roughly the same price, maybe a little less, as the special edition cost brand new.
I have a few. Conquest definitely isn't messing around.
1) Always have Azura on the map. Especially in the timed missions or ones where an enemy mage has "freeze", being able to move twice in a turn is more useful than it's ever been in these games. It's very easy to level her up by just having her sing on every turn. Have her sing for someone even if moving someone on a second turn isn't necessary - it's easy exp.
2) In Awakening, "pairing up" was almost a win button. In Conquest, pairing up will only give you a defensive boost or your partner possibly defending you - they will not attack with you. Have them stand side-by-side and they will give you an offensive bonus and possibly attack with you, but they won't throw themselves in harm's way. For the most part. "pairing up" is only useful if you intend to use a weaker unit or a support unit as a shield when your squad is cornered, such as on a survival mission. Alternatively, you could use a high-level unit as a shield when trying to level a lower level unit.
I suspect a common mistake is that people would pair up units too often and the lack of exp to the supporting unit as well as being unable to utilize each party member will put you at a disadvantage. Also, be mindful of the new color triangle as well as the traditional weapon triangle! It's not enough to just look at axe/lance/sword anymore!
3) Wherever possible, let a low-level person or support unit (even Azura) get a kill. In a game where you can't level grind, getting everybody as equal as possible in levels and promoted to an advanced class is super important.
4) Maids, Butlers and Adventurers ftw! Definitely do your best to keep Felicia, Jakob and a promoted Niles alive. They are so useful because maids & butlers can throw daggers AND wield staves. Adventurers can fire arrows and wield staves, but can open chests too! Always have these people standing right behind your front-line units.
and lastly
5) It's not always ideal or possible, but in general this type of formation is usually good
Frontline - Heavy units like knights or melee fighters like Berserkers and Heroes.
2nd line - Cavalry units and ranged units. When an enemy is killed, your horsemen can quickly move in and score another kill either on the enemy frontline (where your cavalry unit will likely be standing next to a heavy unit for an offensive bonus) or attack enemy archers etc. a few spaces back.
Back row - Healers, songstress and fliers.