Finding combinations of 3 numbers
Your current code
After a long investigation of the problem, I ended up solving more than what you are asking for. I totally misunderstood what your code were actually doing. I found out that your $numbers array is not interesting at all.
The output of your code, the variable $x is the Binomial coefficient for 6 choose 3. (Which is quite funny because in your question you asked about 7 numbers, but in your code you are using 6).
Your code is doing a whole lot of array_pop, next and prev which is just ugly. You need to spend more time thinking about what the algorithm is behind all of this.
Additionally, I don't know what you are intending to do with these lines:
$threes[] .= next($number);

Why use .= ? Last time I checked, .= is for string concatenation. What does that have to do with anything? And why do that when you add a new index to the array? That code does exactly the same as $threes[] = next($number);
You seem to add a whole lot of stuff to the $threes array, but you never end up using it.
Additionally, your $stake, $odds and even $slice seem totally irrelevant here.

Shortening your code
When removing everything related to $threes, and simplifying your multiple $x++ to one $x += ...; your code can be dramatically shortened:
&lt;?php

    $numbers = [2, 8, 16, 30, 44, 48];
    $number  = $numbers;
    $x       = 1;
    echo implode(&quot; | &quot;, $numbers).&quot;&lt;br&gt;&quot;;

    foreach ($numbers as $key) {

        if (count($number) == 6) {
            $x += 10;
        } elseif (count($number) == 5) {
            $x += 6;
        } elseif (count($number) == 4) {
            $x += 3;
        } elseif (count($number) == 3) {
        }
        array_shift($number);
    }

    echo &quot;Total combinations of 3's: &quot;.$x.&quot;&lt;br&gt;&quot;;

?&gt;

Then, when removing some more fluff and using an integer variable instead of that $numbers array.
&lt;?php

    $number = 4;
    $x       = 1;
    for ($i = $number; $i &gt;= 0; $i--) {
        if ($i == 6) {
            $x += 10;
        } elseif ($i == 5) {
            $x += 6;
        } elseif ($i == 4) {
            $x += 3;
        }
    }

    echo &quot;Total combinations of 3's: &quot;.$x.&quot;&lt;br&gt;&quot;;

?&gt;

So, your code is not very extensible in it's current form. It can only support 3's and a maximum $number of 6... That's not optimal.

A better approach
This is how I calculate the Binomial coefficient in Java - which is in use in my Minesweeper Flags Extreme project, translated into PHP
function nCr($n, $r) {
    if (($r &gt; $n) || ($r &lt; 0)) {
        return 0;
    }
    if (($r == 0) || ($r == $n)) {
        return 1;
    }
    
    $value = 1;
    
    for ($i = 0; $i &lt; $r; $i++) {
        $value = $value * ($n - $i) / ($r - $i);
    }
    
    return $value;
}

echo nCr(8, 4); // Prints 70
echo nCr(6, 3); // Your code example, prints 20

It is very tempting to write $value *= ..., but avoid doing that as the results will not be the same in languages which separates integer division and floating division (not that PHP is one of those languages, but I would still recommend avoiding it).