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 Spanish 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 Spanish.
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.

¿La has visto, a Maria?
[Have you seen Mary?]

click to enlarge

In this example, there are three prosodic words: La has, visto, and a María, assembled in two intermediate phrases (ip): La has visto and a María.
The first pitch accent is beared by the auxiliary verb has, the second one is aligned with the stressed syllable vis- of the past participle visto, and the third one is on the stressed syllable -rí- of María.

The three PrWord are separated from one another by break indices at least of level 1. Between La has and visto, there is no perceived break and they are separated by a BI 1, but between visto and a María, the prosodic rupture is higher: there is an ip boundary, which is tonally marked by a H- boundary tone on the last syllable of the first ip, -to. This rise cannot be attributed to the pitch accent on the stressed syllable since it is a H+L*, so it must be interpreted as a H- boundary tone.
Boundary tones only occur at breaks of level 3 or 4. Here the perceived disjuncture in not very strong: La has visto is an intermediate phrase. So the boundary between the two first PrWords La has visto and a María has to be labeled with a BI 3.