They will sound like triplets because you are playing them as triplets, easy answer don't play them as triplets...but how?
Because you use three fingers your your brain assumes 1-2-3-1-2-3-1-2-3 so your fingers play 1-2-3-1-2-3-1-2-3. Change the way you play and feel to counting 1&2&1&2&1&2 etc.
In this simple mind game you do not learn to associate 3s as 3s but as 2s.
1-2-3 is associated as three very distinct syllables using numbers that reinforce the idea of triplets. But 1&2 is still three syllables 1-&-2 using only two numbers, so when counting because the '&' is a link to the numbers we know as being 1-2 we will learn to not work in three but in twos by linking them with the '&'.
Below are examples of a triplet 1-2-3 by counting in threes or 1&2 when assuming them as two. Depending on your three finger style this new thinking will also help educate the plucking hand to be 1&2 a lot easier and faster because your brain is not getting in the way.
It shows better when used in longer bars so here is 12 bars of four in each;
1231/2312/3123/1231/2312/3123/1231/2312/3123/1231/2312/3121/
1&2&/1&2&/1&2&/1&2&/1&2&/1&2&/1&2&/1&2&/1&2&/1&2&/1&2&/1&2&/
As you can plainly now see in counting in threes the 1st beat of each bar changes over three so the 1 is at the start of each new third bar, in effect the 1 is moving around, that strong beat number we associate with being in time is not solid to the ist beat of the bar, so to find it we would take the easy route of 1-2-3-1-2-3 in effect make tripets, two sets of three, to fit any 4/4 based structure.
So for you the above example feels
123123/123123/123123/123123 rather than 1&2&/1&2&/1&2&/1&2& as it wants to be.
I said earlier depending on your three finger style, so a quick pointer on that.
Two basic styles, alternating and following, alternating is index middle ring, middle,index, middle ring, middle etc.
Following is index, middle, ring, index, middle, ring, etc.
Even if you use the same patterns reversed by starting on the ring finger the principal is the same, makes no difference.
What you will notice is for the fingers in the following style the '1' moves from index to middle to ring and back to index.
This you internalise with the 1&2&1&2&1&2& etc very slowly so your fingers learn to put the correct feel of a strong beat on the '1' with the correct finger, rather than the leading finger all the time.
If you play alternating then there is no real problem because the '&' is always on the middle finger, and the '1' is always on the leading finger, so i effect this reinforces the notion of it being staright because the lead finger is always on the '1'.....the same as a two finger alternating style.
There me be some issues with the fretting hand as it may have pattern association problems with fingers being used to pluck, but slow playing or finger exercises will sort this out.
Playing and practicing slow is the key.
You have to allow the brain to understand what it wants the fingers to do and how to count this movement. Once learned (internalised) the thinking will be not getting in the way and you just make it faster or slower to suit.
Please note, three finger is not really faster then two, just more efficient, and more dextrous.