I cannot find how to install a third 3.5 inch hard drive in bay 3 of an 8300 microtower

[ILASER]

In the last few days I purchased a second-hand HP Compaq Elite 8300 Microtower PC.  (I plugged it in and it works fine, and fast.)  It came installed with an SSD and a 3.5 inch hard disk in bays 4 and 5.  My object is to replace that hard disk with my two "old" 3.5 -inch hard disks from my previous computer.  The hard disk that I am replacing was in bay 5;  I just pulled it out, reinserted its 4 guide screws in my old disk, and slid it into bay 5; no problem.  I wish to install my second old disk in bay 3.  I have the relevant manuals, but the procedure is so unclear that I am at a loss.  I would greatly appreciate some guidance, such as a step-by-step procedure,  I might add that all four "extra" blue guide screws (6-32, I believe) supplied by HP are apparently being used to hold the existing hard disk.  But all the spare M3 screws are in where HP placed them, but they are no suitable for my disks.
I would appreciate guidance, because the computer cannot serve my needs, and I do need this computer.

[savvy2]

so this is 100% pure physical drive question, no wiring or software or boot drive questions,

just how to stuff it and and screw it down.

well you need the HDD HP drive caddy, sets.  to put in a drive, in the 3.5" bays.

or is this a 5inch DVD drive bay , adaptation questions.

wish i could see inside your PC as things might be missing or wrong

I guess full size tower, sure. bay 5 yes.

some PCs used trays and caddies.

others use what I call articulated slots in the cage and those 3..5"  uses USA screws not metric ever, they use 6-32 screws USA imperial measurements  spec. (hdd invented by USA and IBM) infact, AL Shugart   and screws same.   (RIP)

do you have the 8300 service manual in hand it covers all this but not how to fit 3.5" drives in 5.25inch bays,

page 49, hdd istalll

Install guide screws on the sides of the drive. The type of screws used depends on which drive

bay is being used to house the drive. If you are installing a 2.5-inch drive, you must install the

drive in an adapter bracket.

then

f you are installing a 3.5-inch drive in either of the lower two bays (bays 5 and 6), install

four 6-32 silver and blue isolation mounting guide screws (two on each side of the drive).

dvd are metric

Floppy same 3.5"(sony/panasonic japan rules)

.2.5" metric m.3

3.5": USA imperial 6-32 screws size 6 , 32 threads per inch, not PITCH as ISO metric does.

5.5": can be either metric or 6-32 relic HDD, but most are metric, M.3 ,Philips and Sony licensed it as a CD-ROM , origin. 

2.5"needs special tray. and mostly metric; m.3   generic trays work,.

'

my wild guess PC you have lacks the special blue coded screws . (acts of others)

the magic of articulated mounts, HP,  are genius, IMO  (the guides on some look like  roller coaster track)

https://www.manualslib.com/manual/441557/Hp-Compaq-Elite-8300-Series.html?page=49#manual

see my one glance answer here,  simple , no? (photos are amazing) CMT guessed for case.

[savvy2]

get more screws off ebay.

folks do strip PC and sell vast parts there for any old common PC.

endless there. in fact.

8300 CMT is other name.