Is there any way to downgrade old nintendo 3DS runninig 11.5 stock firmware to 11.3?

You no longer need to downgrade, but you do have 3 options
NTRBoot: This will require you to buy a compatible flashcart and install an exploited firm on it order order to hack your system. Basically this does have some options to it as there are users selling pre-exploited carts on the forums as well the new NDS file has been released. So you can basically exploit your cart using an old NDS(lite/phat.) Regardless this method does need either an R4i Gold (url on cart: r4ids.cn) or an Acekard 2i that has be exploited by either using an already hacked 3DS or a DS(lite/phat) This method also requires a strong enough magnet in order to trick the system into sleepmode
Hardmod: Which requires soldering skills and a few parts in order for it to work
DSiwarehax: This requires an already hacked system to install the exploit into a DSiware game and transfer that to your target system.

 

Its not possible to downgrad, but it's not required anymore, since every firmware can now be hacked. Just go with NTRboot or hardmod. NTRboot is still cheap to do and from what I can tell, you'll soon be able to do it without another pre-hacked 3ds.

 

@BetterDuck What? Of course you can downgrade. You can do whatever the heck you want to now (and recover from it, if "whatever you want" causes a brick ). The question would be WHY would you want to? (other than the Gateway team is slow as hell)

 

Oh shit, I'm retarded. I didn't think about downgrading through NTRboot. I really should avoid these forums when I'm tired. I guess I was dead wrong here.

 

If you are on 11.5, then there's simply no way to downgrade as there's no kernel exploit. Although you really shouldn't downgrade anyways because it's no longer a needed step. You either need to go with a hardmod or dsiwarehax (which requires another prehacked system)
Otherwise you are going to have to wait for a kernel exploit for 11.4+

 

Well, I'm sure lasthax will be released sometime within the next year and as far as I can tell, it only needs Flipnote Studio 3D, so you may want to wait for that.

 

The flipnote exploit at this time does not have a definite answer to if its just a userland or a kernel. It ifs just userland then its gonna be useless for putting full CFW on any 3ds system. Same with rpg maker player

 

A kernel exploit that effects 11.4 (and most like 11.5 because 11.5 didn't add a new NATIVE_FIRM nor much in the line of security.) Basically the same exploits that you used to find with previous methods of hacking, such gaining the ability to downgrade your system to an exploitable OFW.