Quare Sacrosanctum Concilium, de fovenda atque instauranda Liturgia quae sequuntur principia censet in mentem revocanda et practicas normas statuendas esse.
Inter haec principia et normas nonnulla habentur quae tum ad Ritum romanum tum ad omnes alios Ritus applicari possunt ac debent, licet normae practicae quae sequuntur solum Ritum romanum spectare intellegendae sint, nisi agatur de iis quae ex ipsa rei natura alios quoque Ritus afficiant.
Vatican Website Translation:
Wherefore the sacred Council judges that the following principles concerning the promotion and reform of the liturgy should be called to mind, and that practical norms should be established.
Among these principles and norms there are some which can and should be applied both to the Roman rite and also to all the other rites. The practical norms which follow, however, should be taken as applying only to the Roman rite, except for those which, in the very nature of things, affect other rites as well.
Slavishly Literal Translation:
Wherefore the Sacred Council judges that the principles that follow concerning the promoting and renewing of the Liturgy are to be recalled to mind and practical norms are to be established.
Among these principles and norms there are some which can and ought to be applied to the Roman Rite and to all other Rites, [but] it is correct that the practical norms that follow are to be understood as applying only to the Roman Rite, except for those which from the very nature of the thing affect other Rites as well.
Article three is a transition from the doctrinal preamble appearing in articles 1-2 to the treatment of individual topics concerning the reform/restoration/renewal and cherishing/promoting of the Liturgy that will appear in the following chapters.
I think it is important for two reasons. (1) It sets up the pattern that will re-occur throughout the document of proffering doctrinal foundation for the particular practical norms, guidelines and exhortations the document offers. (2) It makes clear that, although the doctrinal foundations apply to all Catholic liturgical Rites, most of the practical norms, guidelines, and exhortations will only apply to the Roman Rite. This will be further developed in the next article, where I hope we will discuss why the Fathers felt it was necessary to declare the equal worth of the various Rites.