Hello, 仍然 means 'still' or 'yet' or 'up to the present' in particular : Example: 他 仍 然 保 持 着 老 习 惯 ta rengran bao chi zhe lao xi guan. He still sticks to his old habits. 她 仍 住 在 伦 敦 。 ta rengran zhu zai lun dun. She still lives in london. 依然 means 'still' as well or specifically 'as before': 她 依 然 像 1 0 年 前 那 么 漂 亮 ta yiran xiang 10 nian qian na ma piaoliang. She's still as beautiful as she was 10 years ago. 还是 could also mean 'still' ,'nevertheless' ,but also means 'or' when linking alternatives as in: 你 要 茶 还 是 咖 啡? ni yao cha haishi kafei ? Would you like tea or coffee?

I think it would be helpful via the following sentences : 1、Would you like anything else to eat? 你想要其它吃的吗？ under the circumstance, we usually add the word "还“,rather than "仍然”， in the front of the word ”想“. so it is changed into this 你还想要其它吃的吗？ 2、He is very young as before.他仍然很年轻。in this case,we only use 仍然 to express. If we use 还 instant of 仍然， it means He is so young that he can live for a long time. hope these idea will work , good luck

依然表示和以前一样，没有变 仍然比依然多了一个意思，就是 恢复原状，比如:他把信看完，仍然装在信封里 其实这两个词是差不多的，大多情况下可以相互转换

They are synomy. As a native Chinese speaker, I never think their difference. In generallly, you can use each one as you like. "还是": It is informal. It always appears in speaking. It's often used in chatting. "仍然": It's between formal and informal. "依然" It's formal.

Actually, in many cases,they can be replaced by others.