The simplest answer is that a one card tarot reading will give you a more accurate psychic screenshot of your life.
Each additional card in a layout factors in more complexity by introducing additional data.
It narrows the probability of making correct predictions and insights, because there are just too many boxes that have to be checked for anything to happen in a predictable way.
Some tarot readers use a three-card layout for past, present and future. But what's past is past; there is no useful value in re-examining what has already occurred. The only valuable information that can be gained is to understand where you are now and what relationship that has to where you want to be in the future. So the "past" card is just a distraction that confuses the issue. The one card tarot layout tells you what is important now, and where this leads.
Other psychics use the so-called Celtic Cross layout. This intricate pattern uses 10 cards that supposedly explore the relationships between inner and outer influences, hopes and fears, and advice, challenges and outcome.
The problem with this is that you can't divide your life force this way. It is a completely meaningless exercise that tells you nothing. The most powerful application of psychic energy is one that is laser-focused, rather than scattered. Psychic energy is all about unity, not division.
The one card tarot is a unified representation of everything that is going on with you, and therefore it is much more accurate.
The most important thing to understand, is that each tarot card is a complete story- with a beginning, a middle and an end. When you add another card to a layout, you are adding another story. Its like reading two books at the same time- a word from one, than a word from the other, then back to the first for the next word, and so on.
Wait a minute though. Didn't I just say that each card was a snapshot? Then how can it be a complete story?
Each card does only show a moment, but that doesn't mean that that particular moment arrived out of the blue. Something (or things) led up to it. Likewise, something happens next, even if you can't see it printed out all nice and neat for you. Figuring out what those things are, is up to you, the reader. So, if you get stuck when trying to interpret a card, a good practice is to ask yourself, "What's going to happen next?" Then, try to visualize it happening. Or, (if its easier) ask yourself, "What happened right before this scene?" And then, visualize that happening. Make it real to you. This is how you can use tarot cards to flex your psychic muscles.
Don't form a question in your mind. Your uppermost concern will be reflected in the single card that is selected. Keep your mind as blank as possible, relax and maintain an emotional distance.
If you are wearing glasses or contacts, take them off/out.
Take the cards and lay them face down. Put your hands on them and mix them using circular motions. Your instinct will tell you when it is time to stop mixing them.
Pick the cards up, cut them (divide them roughly in half) and shuffle 3 times using the riffle shuffle method. (Bend them slightly, riffle them to join, make them arch to come together.)
the cards again, placing the bottom half on top of the other half, then take the top card and turn it over. If it is upside down, do not straighten it. This is your one card tarot outlook.
As soon as you turn the single card over and see the picture, you will experience some kind of emotion. This is important! Take note of it, and interpret any information in the card through the prism of this emotion.
Any tarot reading depends on how you "feel" about the card you receive.
Then, notice the specific details of the card that grab your attention, glance away and see which ones remain pictured in your mind. These tell you what you are focusing on in your life right now.
To foresee the impact of these details on your future, look back at the tarot card. Find the pieces that stood out to you initially. Close your eyes and visualize these. Very carefully, try to make them shift or move a bit in your mind's eye. These are indications of how conditions today can impact your future. Take note of any impressions, feelings or thoughts, that come to you.
You gain expertise using the one-card tarot method by understanding that your interpretation is 100% right, every time. There is no wrong way to interpret a card, as long as you approach the exercise with an openness to anything that might be revealed, and a neutral state of emotion. Here is a quick list of tarot card meanings.
Recognizing what you are looking at is trickier. Tarot cards are made up of symbols. These symbols are both disguised (so the uninitiated cannot see them clearly) and amplified so that their effects are felt more strongly. See, How do tarot cards work, for a more complete explanation.
I recommend using the Rider-Waite tarot deck because its creators got the symbols right. Even someone who is not trained to recognize them will feel their power. Unfortunately, this is not the case for many other tarot decks.
For more insight into tarot symbolism and how it relates to your one card tarot reading, see this info tarot section.
For a deeper understanding, check to see if your one card layout is one of these spiritual guidance tarot cards.
To understand what a "marker" card is in tarot, see 10 of cups.
For exercises that will help you develop the skills to project the future applications of your tarot card, see these pages: