I want to use the Mail app on OS X Sierra to manage my emails from 3 different Gmail accounts. But I am using a MacBook Air which is low on space. So is there any way to use Mail app to access my mails without downloading the mails, but only using the app to access the mails on the cloud?
It's not possible to prevent Mail downloading emails.
You can keep Mail from downloading attachments in Preferences → Accounts → select account → Account Information by setting Download Attachments to None. You will then be prompted to download each attachment individually when you request it.
To determine when Mail will delete downloaded attachments, you can go to Preferences → General and set Remove unedited downloads to When Mail Quits. This means when you quit Mail, any attachments you downloaded by clicking on them (if you set the previous setting) will be removed, reclaiming the space. They can still be downloaded again if you choose next time.
To reduce the number of emails available to Mail, you can configure the number on the server. In Gmail settings → Forwarding and POP/IMAP, set ‘Limit IMAP folders to contain no more than this many messages’. Only number of messages selected here will be available for Mail to download.
You can also hide emails with certain labels from Mail using Gmail settings → Labels and unchecking Show in IMAP.