So I've made my first deck, but I have a question. My heroes are Boromir (Leadership), Theoden (Tactics) and Beravor (Lore). Now, they each come from different spheres and I was wondering, is making decks with multi-sphered influences a good idea or not. Or is it better to create decks with a singular sphere?

I would not say that any particular strategy is "better" or worse as much as each has plusses and minusses. A mono color deck can more easily pay for expensive cards (4 or 5 resources), but they lack flexibility. Multicolored decks can put cards in that do everything from combat to questing to treachery cancellation to card draw to location control to healing, but it is a lot harder to pay for cards that cost more than 2 resources. You hero selection, in my opinion, is very poor for a 3 color deck, primarily because of tactics theoden. He has a very high threat cost, which will cause you to threat out more quickly. In order to justify his high cost you have to make significant use of his ability (which you can't because you have no other tactics heroes) or his stats (which you can with some ready effects from different spheres). I would say you can't do that enough to make the deck work. This is true because leadership Boromir also has a high threat cost, which requires you to dedicate a lot of deck space to supporting allies in compensation. Most of the time people use only one hero with such high threat and then dedicate deck space to that hero. Then they use lower threat support heroes.

Bottom line, the real issue your deck has is not that it is three colors but that it has heroes that don't go well together.

Maybe Logolas tactics would be better : lower threat cost, and is ability.

I would not say that any particular strategy is "better" or worse as much as each has plusses and minusses. A mono color deck can more easily pay for expensive cards (4 or 5 resources), but they lack flexibility. Multicolored decks can put cards in that do everything from combat to questing to treachery cancellation to card draw to location control to healing, but it is a lot harder to pay for cards that cost more than 2 resources. You hero selection, in my opinion, is very poor for a 3 color deck, primarily because of tactics theoden. He has a very high threat cost, which will cause you to threat out more quickly. In order to justify his high cost you have to make significant use of his ability (which you can't because you have no other tactics heroes) or his stats (which you can with some ready effects from different spheres). I would say you can't do that enough to make the deck work. This is true because leadership Boromir also has a high threat cost, which requires you to dedicate a lot of deck space to supporting allies in compensation. Most of the time people use only one hero with such high threat and then dedicate deck space to that hero. Then they use lower threat support heroes.

Bottom line, the real issue your deck has is not that it is three colors but that it has heroes that don't go well together.

My overall deck doesn't really have a heavy tactics presence anyways (I think perhaps only 8 cards out of 61), so I had thought of perhaps switching out for either another Lore or Spirit hero.

A Lore hero allows you to put more trap cards in your deck (if you have them), they're a ton of fun!

This Warden of Arnor article is about how the spheres combine and what each combination results in, I found it to be very spot on and a good read when I stumbled upon it a while ago:

https://wardenofarnor.wordpress.com/2015/07/12/how-do-the-spheres-combine/

My overall deck doesn't really have a heavy tactics presence anyways (I think perhaps only 8 cards out of 61), so I had thought of perhaps switching out for either another Lore or Spirit hero.

61 cards is a big deck. You should really try and get it as close to 50 as possible so your deck is more consistent.

There are some good deck building guides here: https://talesfromthecards.wordpress.com/deck-building-101/

Ok so one thing that I don't think anyone has brought up yet is how many people you are playing with. You see, the reason for using more than one sphere is that the different spheres are good at different things, as you probably very well know. So while it doesn't make that much sense to play mono tactics solo, it can be quite powerful when there are 2 plus players. The other thing that is best to remember is that you ought to utilise all the hero abilities. This often means focusing your deck on a certain trait, whe there or not the spheres work out perfectly. So it really depends on what quest, how many players are questing with you, and traits whether or not curtainow spheres ought to be used alone or together,

Very true. I play two handed and use a tactics/leadership deck and a lore/spirit deck to cover all four spheres and provide a wide variety of player cards to deal with any situation. I find that tactics/leadership is a great combination for a combat deck and lore/spirit is a great combination for a questing/support/manipulation deck.