Break indices of level 3 (BI 3) correspond to the end of minor prosodic units: they mark the boundaries of intermediate phrases (ips).

There are several arguments in favor of a level of an intonationally-defined constituent underneath the intonational phrase: the intermediate phrase (ip).

The first argument is a perceptual one.
In Catalan prosodic transcriptions, transcribers clearly distinguish between two levels of degree of perceived disjuncture. The end of the weaker disjuncture corresponds to a level 3 break index in the ToBI system, while the strong disjuncture corresponds to a level 4 break index.

The second argument relates to intonational marking.
Like the intonational phrase, the intermediate phrase is tonally marked after its final pitch accent (yet not as strongly as the intonational phrase), but the inventory of boundary tones that appear in this position is of a different (but partially overlapping) class.
Typically, H- boundary tones, also called “continuation rises”, mark the end of an intermediate phrase.
Frota, D’Imperio, Elordieta, Prieto & Vigário (2007) examined the phonetics and phonology of prosodic boundaries in such positions in five Romance languages, among them Catalan.
They found that the two dominant boundary tones in statements (located at the end of the first prosodic constituent) were either the continuation rise or the sustained pitch.
By contrast, intonational phrase-final edges were generally signaled by a low boundary tone (L%) and by a wider inventory of boundary tone combinations.

On vas, com hi vas i quan tornaràs?
[Where are you going? How are you getting there? When will you be back?]

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In this example, there are six prosodic words assembled in three intermediate phrases.

The first ip on vas is constituted of two PrWords and ends with a L- boundary tone but it is not followed by a pause.
The second ip is com hi vas, constituted of the two PrWords com and hi vas, ends with a H- boundary tone -that is, a continuation rise- and it is not followed by a pause either. Notice that the last segment, which is a fricative, is voiced since it is between two vowels. This could not be the case if it were in IP-final position.
The third ip i quan tornaràs is also constituted of two PrWords and ends with a L% boundary tone, which is followed by a pause.

Thus, in this example, there are three boundary tones but only the third one is followed by a pause. That means that this sentence contains one IP, made of three ips. The first two boundary tones that are not followed by a pause correspond to BI 3.