Break index 3 marks a strong degree of disjuncture between adjacent words, or between a word and following silent interval. This is the strongest boundary marked in the break index tier of a J_ToBI transcription.
This break index often corresponds to the boundary of the tonally-defined intonation phrase, the highest level of the prosodic hierarchy of Japanese (see [Venditti (forthcoming)]). The intonation phrase is the prosodic domain within which pitch range is defined and thus within which downstep occurs. At an intonation phrase boundary, the speaker resets to a paradigmatically contrastive new pitch range value for the next phrase. This also is the unit at whose edge a H% (or HL%) boundary tone may occur (see section 3.3.3 above). However, while the unit marked by a break index 3 often corresponds to the intonation phrase, the two are not always identical (see section 4.5 below). As break indices are primarily subjective evaluations of perceived disjuncture, labellers should evaluate the strength of each juncture by carefully considering the sound record, and not only by looking at the F0 contour.
Utterance 
sankaku
also shows an example
of the strong disjuncture between adjacent words marked by break index
3. As noted above, this utterance begins with two accentual phrases
/sa'Nkaku no/ `triangle-GEN' and /ya'ne no/ `roof-GEN'. The boundary
between these phrases is marked by BI 2 (medium disjuncture). The
boundary between the second phrase /ya'ne no/ and the next phrase
beginning with /maNnaka/ `middle' has an even stronger sense of
disjuncture, which is marked with BI 3. Tonally speaking, the first
two accentual phrases of the utterance form one larger intonation
phrase unit, with downstep causing the second phrase to be lowered
with respect to the first (see section
3.1.1 above). The pitch range is then reset on the word /maNnaka/,
which is the beginning of the next intonation phrase. The large pitch
rise here between intonation phrases gives the sense that a new unit
has begun, and thus there is a strong sense of disjuncture between
/no/ and /maNnaka/. Example utterances
curtain
and
zettai
also show a pitch range reset
associated with a strong disjuncture marked by BI 3.
A large F0 rise (e.g. pitch range reset) is one factor that can make labellers sense a strong disjuncture between words. In addition, there may be other factors involved too, including segment lengthening, F0 lowering, decreased amplitude, pauses, etc. The speech signal is full of information that can contribute to the subjective evaluation of disjuncture, and the F0 contour is only one thing.
Example utterances 
pinku_mado
and
nibanme
(and others later in this guide)
show the marking of break index 3 before long pauses. Inserting a
pause is one way for a speaker to indicate a separation of information
(i.e. a boundary) in the stream of speech. In addition, all of the
examples presented here are marked with BI 3 at the end of the
utterance. This indicates a strong disjuncture between the final word
and the following silent interval (see section 5 below for
discussion of marking the degree of finality of these pre-pausal and
utterance-final boundaries).