chiddush of netziv regarding shehechiyanu during the 3 weeks

Rama (O.C. 223) paskens that the bracha of shehechiyanu is optional, not a chiyuv. This is based on Eiruvin 40b:

 ואמר רבה כי הוינא בי רב הונא איבעיא לן מהו לומר זמן בראש השנה וביום הכפורים כיון דמזמן לזמן אתי אמרינן או דילמא כיון דלא איקרו רגלים לא אמרינן לא הוה בידיה כי אתאי בי רב יהודה אמר אנא אקרא חדתא נמי אמינא זמן א"ל רשות לא קא מיבעיא לי כי קא מיבעיא לי חובה

The gemara there discusses whether a person has to recite shehechiyanu only on the 3 regalim, or even on R"H and Y"K.  R' Yehudah answers that of course you need to recite shehechiyanu -- if you recite shehechiyanu even on a new fruit, certainly you can recite it on the holidays of R"H and Y"K.  The gemara rejects R' Yehudah's argument.  The shehechiyanu on new fruit is RESHUS, optional.  The question with regards to R"H and Y"K is whether there is a CHIYUV to recite a bracha.  

QED, says Rama, that there is no chiyuv bracha on a new fruit, new clothes, etc.  If you want to celebrate the enjoyment of something new, you CAN say a bracha, but you don't have to.

Netziv in Haamek Sheila on Shi'ilta 171 (p336) disagrees and draws a distinction between shehechiyanu on zman, on reaching a particular milestone in time, and shehechiyanu which is recited on buying new clothes and the like.  Netziv is mechadesh that the bracha on a new fruit is not on the enjoyment of the fruit, like the bracha on a new garment, but rather it is on having reached the season in which that new fruit ripens, a bracha on zman.  

It is only the bracha of shehechiyanu on zman, like a new fruit, which the sugya in Eiruvin proves is RESHUS.  The shehechiyanu on the enjoyment of buying something new is obligatory.

This chiddush of the Netziv leads to a nafka minah during the three weeks.  The S.A. paskens that one should not recite a shehechiyanu on a new garment or a new fruit during this time period.  The logic behind the din is that you don't want to say a shehechiyanu and give thanks for reaching an inauspicious time period.  That sevara makes sense when dealing with the shehechiyanu on zman, on the time period, like the shehechiyanu over a new fruit.  However, the shehechiyanu on a new garment and the like is not a bracha on zman -- it is on the enjoyment from the newly purchased item.  There is a separate din, says the Netziv, not to buy new things during the three weeks so m'meila you have nothing to recite shehechiyanu on, but if a new items falls into someone's lap, e.g. they got an inheritance or won the lottery, then according to the Netziv, a shehechiyanu would be recited.