That entirely depend on how you chose to play your deck. I will try and give a few example to help you out.
First let us look at an undead deacy deck. Similar to Irva's Undead (linked below). In this deck, Irva is very important. You could chose to run 3 Irva's, but over a long time, you will likely find, already having an irva on the board, you have dead cards cycling through your deck. Choosing to have 2 I find, is acceptable because she will always be at the back of a row, and not in danger. She is out of burn range. Overall she should be safe.
If you compare her (irva) to Reloras in a tempo order deck. They are similar, but, Reloras will often be played in a middle row to take advantage of his Resonance ability. He is in alot more danger, do to primarily being on the middle row. I find it is often better to run 3 Reloras (depending on how much your deck depends on him) because he will die often, and needs to be played several times in a game.
My third example is a Mid game growth deck. Mostly middle cost creatures, but also runs god hand and Jarl Urhald. While Jarl is a powerful creature, I am nearing the end of my curve. I want to end the game as quickly as possible. If my Jarl Urhald dies, likely my game is over. I makes more since to run more God Hands then 2 Jarls, because, I will (almost) never play Jarl twice in one game, as when Jarl is played I need to win the game quickly to survive. Therefor it would be illogical to run 2 Jarls, if i will only ever play one.
Now in answer to your question. As said before, It all depends on your play style. If you run 2 theas, and protect them as much as possible when in play, then you should be good. However If you like running Thea at the front of your row, and using her as a wall, she is likely to die, and you may want to have 3 in your deck. However this only works If you have a late stalling curve, and Win late late game, you could consider 3 theas. Just be careful not to *Unique kill, and good luck! Just keep experimenting and see what works for you.
(decay deck - {"deck":"","author":"Ikraen","types":[163,163,164,164,349,349,349,179,179,268,268,181,181,265,265,350,350,350,183,183,172,195,195,195,362,362,177,177,353,353,271,271,200,200,352,352,382,59,59,194,194,343,381,352,271,382,268,164,194,381]} )
_________________ I have come to the borders of sleep The unfathomable deep Forest, where all must lose Their way
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