Whether I can use scatter instead of web3 metamask? In this way I do not need to add the metamask add on on my browser? And I can run the dapp on mobile for scatter because metamask does not support mobile?
web3 does not require MetaMask. Web3 can be used with any web browser, as long as you have a node that exposes Web3 API (over http RPC or WebSocket).
Alternatively, you can get a free API endpoint from https://infura.io/