Fancy goldfish are prone to swim bladder issues related to several different causes... some minor, some more serious. The wrong foods can cause digestive bloating and/or constipation and this can put pressure on either of the swim bladders or the lines leading to them and this prevents the fish from being able to control their swim bladders. Try feeding green pea (pinch the shells off and discard the shells.. then mash the *meat* a little if your fish are really small. This will help cure constipation issues.
Other causes can be internal bacterial or parasitic issues causing inflamation of digestive tract or other organs causing the same pressure on the lines or swim bladders.
Fancy goldfish are prone to this because of the inbreeding that took place to create the deformities and mutations you can see on the outside also can cause deformities and mutations to internal organs as well. Further, their two swim bladders, which are normally spread out far away from each other in *normal* goldfish, are all crammed together in fancy goldfish due to their compacted and round body shape.
As Rbishop already said, your 70 liter (around 20G) tank is too small for goldfish... even one goldfish grows too large for that size tank. Are your other two fish also goldfish?
Start doing daily 25% PWC's (partial water changes) as dirty water will also cause fish to get sick much faster so giving them clean water will help them get well faster.
Give us more info about your water parameters (ammonia, nitrite, nitrate, pH, temperature, etc.)