In Excel is easy: =(A2/1000/3600)*"01:00:00"

Set for cell B2 own format number (for 3 decimal places) : hh:mm:ss.000;@
or
Set for cell B2 own format number (for 1 decimal place) : hh:mm:ss.0;@       (Round up for one decimal place)

Let me rephrase that last question:
"My question here is how do I right the equation so that any number of milliseconds less that 172,800,00 equals the right number of days?"

I meant to type:

My question here is how do I right the equation so that any number of milliseconds more than 172,799,999.00
equals the right number of days?

This is a very helpful discussion for me. I found Rob Bell's and Jan Hook's previous posts such a great help to the initial tutorial. I needed the additional "day" column to count out my milliseconds which were over the 84,000,000 per day. So I modified Robs easy equation to say, "=TEXT(A1/86400000,"dd:hh:mm:ss.000")." I also used Jan's second equation posted. Funny thing though, on google sheets, when I use the expression on 0ms, the result is 30:00:00:00.000, instead of 00:00:00:00.000. When I try the expression on 1ms, the result is 30:00:00:00.001, instead of 00:00:00:00.001. When I try the expression on 99999999ms, the result is 31:03:46:39.999. When I try the expression on 1111111111ms, the result is 11:20:38:31.111. Notice that all of these except the 1111111111ms begins with a "3", such as 2ms=30:00:00:00.2. It seems to me that strangely the expression "=TEXT(A1/86400000,"dd:hh:mm:ss.000"), and Jan's longer version both result in throwing off the math by exactly 30 days, until the 172,800,000ms, at which time the 30 days drops off, and the expression reports a result of 01:00:00:00.000.

My question here is how do I right the equation so that any number of milliseconds less that 172,800,00 equals the right number of days?

You can get the original formula to work by modifying the millisecond portion of the formula to include the text function to format as follows: =CONCATENATE(TEXT(INT(A1/1000)/86400,"hh:mm:ss"),".",TEXT(A1-(INT(A1/1000)*1000),"000"))
If you need to allow for days you can do the following:
=CONCATENATE(TEXT(INT(A1/1000)/86400,"dd:hh:mm:ss"),".",TEXT(A1-(INT(A1/1000)*1000),"000"))

this produces some errors in some cases (I've not bothered to figure out why or in what cases, but it messed up in about 10% of my rows. Instead, divide the millisecond by 86400000, for example with

=(A1/86400000)

then apply a custom number format to the new value, such as:

[hh]:mm:ss.000

..this worked fine for me in 100% of my rows.