'99 GT convertible - 5sfe engine

Having problems shifting into 3rd gear. When shifting into 3rd, I have to really press the clutch all the way down and its difficult to shift into 3rd meaning I have to use more force to get it into 3rd than other gears. Its almost like 3rd gear is stuck or blocked from shifting into it.

None of the other gears are bad. Car has 125k miles.

I was wondering if I need to replace the clutch, replace transmission fluid, and/or do a transmission flush.

Any ideas what the problem could be and what the solution is?

Most likely it's the notorious third gear synchronizer. Fluid change might help, I hear a lot of people on here use Synchromesh instead of regular 75W-90. Easy way to test to the clutch is get up to 30 mph and throw it into 5th, if the RPM's go up and your speed doesn't the clutch is starting to slip. If the RPM's and speed go up evenly the clutch is ok.

so shifting from 1st to 2nd isnt hard, or 3rd to 4th? is it going from 2nd to 3rd or 4th down to 3rd?

if its specific to 1 gear, ill say synchros. but if its also hard on other gears, ill say clutch.

I'm getting the same issue, I got a 97 convertible and shifting to 3rd sometimes makes a slight grind sound,, well that and my clutch slips,,, just trying to ride it out right now.

well if the clutch is slipping then you know it needs to be changed. but for a slight grind going into a gear. thats a synchro issue

Thanks for all the responses. Sounds like it maybe a synchros issue.

what is the most cost-effective solution? Does the whole transmission need to be replaced/rebuilt?

I'd try switching out to Syncromesh first before shelling out for a partial rebuild.

I had an issue in my gen 5 that never allowed me to shift into any gear from neutral. Downshifting was not possible either, even if I shifted too slowly, it wouldn't work. I bought the car for $500 so I had zero complaints and was my beat around anyways. I learned real quick how to double clutch and it worked! Downshifting became possible and shifting out of neutral became possible.

But more specifically to your issue:
When shifting into 3rd try not forcing it. Push as hard as it will go without grinding or making the shifter shake/jump too much. Then slowly let off of the clutch and hold low and steady on the throttle. She should pop right in. I use the same trick when going into reverse. Once you get that down you won't even notice there's an issue. It's definitely best to fix it but this is a good meanwhile work-around. Drove my dad's dump truck a while back and this is how you had to shift into every gear.