Depending on the individual personality of each fish you could have problems, but the larger the tank the less likely their will be problems. The problem will more than likely come from your Flowerhorn and your Clown Knife. Flowerhorns have a strong tendency to be very aggressive to its tank mates. I had a little 4-5 inch one destroy a thermometer. In the wild the Clown Knife can get larger than 3ft, but in a home aquarium my guess is probably around 2.0-2.5 feet. You would probably need to get at least an 180gal (6x2x2) size tank to house all three together for life. You could get a smaller tank say a 125 or 135 and keep all three until the Clown Knife gets to about 18-20 inches at that point you would need to trade it into your LFS if they take trades or find another aquarist that would take it for you. You have the chance of putting them all in one tank and one day finding two dead fish and one greedily swimming around by himself. You are dealing with one fish that gets very large and one that can have the potential to be murder. My suggestion is get a 125 or 135, keep all three of them in there, but keep an eye on them and when problems arise or when the Clown Knife gets to 18 inches remove the Clown Knife and get a few more large tinfoil barbs.