$a=[
'0'=>[
'a'=>'1',
'b'=>'1',
'c'=>'1',
],
'1'=>[
'a'=>'1',
'b'=>'2',
'c'=>'2',
],
'2'=>[
'a'=>'3',
'b'=>'3',
'c'=>'3',
],
];

$tmp = [];
foreach($a as $record){
$valA = $record['a'];
$valB = $record['b'];
$valC = $record['c'];

//a 的值做 key
$tmp[$valA][] = [
'b' => $valB,
'c' => $valC,
];
}

$res = [];
foreach($tmp as $key=>$val){
$a = $key;
$c = [];

foreach($val as $part) {
$c[] = $part;
}

$res[] = [
'a' => $a,
'c' => $c,
];
}