$m=gc c:\temp\matrix.txt
$array=@()
$m | Foreach{
$elements=$_.split("`t")
$array+= ,@( $elements[0],$elements[1],$elements[2])
}
$array[0][0]
$array[1][2]
note the comma (line 6) that force powershell to consider the added line as an array
To ignore the first line you can do something like this
$matrix=""
(1..($m.count-1)) | foreach{ $matrix+=$m[$_]}