Bit of a crunch changing down into third,can i do anything easy to help it?

The reason they work with the engine OFF is that they are not spinning so there is nothing stopping them moving in and out of position.
Ill need a bit more information to give you more help.
Is this a manual or an automatic vehicle? Does it do anything else when the engine is running? Does it make noises like grinding or crunching? Does it jump roughly as it goes into gear or slip into gear? Please comment below and I'll try to help you more.

U joint their part of the drive shaft assembly lets it move up and down when you go over a bump
Makes a scarry noise when they are bad but they are cheap 10 bucks a peice and as long as you don't drive it for ever it won't fail tomarrow but get if fixed soon .

20/50 is fine unless you in a winter in alaska ,use cheapest oil you can buy an and change it every 6000miles with a oil filter every third oil change ,Do not use long life oils with silly oil change periods as this sort thing cause the engine to clag up inside with depoisits then when these deposits become thick enough they break off and clog the oil pump pick up pipe filter and then it has no oil and the big ends go bang bang crunch

sounds like it may be the sycnros that have seen the end of there life requires opening the tranny and replacing

Hi
It is the syncromesh, inside the gearbox, difficult to repair, most often you need to replace the gearbox although someone who was familiar with Renaults be able to, is yours the hydrolic clutch or cable??

If its the cable type you may be able to adjust it so it does not pop out of gear, but you will always have the little crunch coming down into third.

Regards
Dan

Yep, there are lots of guides out there on eBay or similar, personally I like the one written by TD5Alive, but they're all mostly the same. It boils down to cutting a good bit of metal out of the existing structure of the front end, and then mounting the new light clusters & eyebrows.
Not 100% sure if I'd be happy doing it to mine TBH, it does seem a bit "bodgy". May just buy the front end off a crunched one (that still has a front end of course!) and use that...

Easy DIY is to change all fuel, air, and oil filters, do full tune-up, and change ignition cables, too. See if problem goes away. Take a look at coil, too, those cars have a known coil issue and it needs replacement every 25,000 miles

do you need new brakes

The possiblities that come to mind are
1 Bad synchro in transmission 2 bad bearing(s) in transmission input shaft.   3 Bent shift linkage or shift fork (I doubt this, but it's possible)
Only number 3 is relatively cheap.  I'd go back to where/who you bought the car from, if it was that way when you bought it, and ask them to help.  It can't hurt.

Using force is NOT the solution.  If the syncros are bad, until you get them replaced, you'll be better off to determine the RPM difference between 2&3 gears (maybe 800-1000?) and also 4&5 gears (maybe 500-800?) and learn to put a modest amount of effort at exactly the time the rpms fall into that diffference.  
Example Say the difference between 2nd gear at 3500 RPM and 3rd gear is 1000 RPM, then when you take it out of 2nd for an upshift to third,  delay until the RPMs are just getting to 2500. If you are on a budget and can't fix the transmission (it's going to be big bucks) you may find you can get pretty good at this.    It is possibly to shift a car this way without even using the clutch, although I don't recommend this and it takes a great deal of practice and luck.
While you have this problem, don't downshift into 3rd,  just run the RPMs down in 4th (say to 2000 or less) until you can shift into 2nd at an acceptable RPM. The least amount of shifting you can do until you resolve this the better.
Another choice until you can fix it, is to just run up the RPMs in 2nd and then shift to 4th, and then forget you have a fifth gear.  This is much better than forcing, it's possible to do more damage by forcing. 
Is it hard to get into 3rd from 4th AND 2nd?  
..ajm

hi if the gearlever is all over the place start by checking the remote control rods to the gearbox see if any of the bushes have worn if they are ok check all the mountings that hold the remote assemabley check the rear engine and gearbox mountings if all these check out then the problem is in the selectors in the box if it crunchs when changing down the syncromesh bulk ring has worn and or the complete hub it sounds like the gearbox needs overhauling next check you have no lights out brake lights as well if ok check all the connectors to the lights one at a time start at the back and check for bad grounds on the lights what mileage has it done paul