Yes, absolutely. Even if only a symbolic portion of the ashes (Asthi) is available, immersing that portion with full faith and proper ritual at the Triveni Sangam is highly beneficial and meritorious. The divine rivers accept the offering with the Bhavana (intention and emotion) behind it. The scriptures mention that even a single bone fragment immersed in the Ganga (and by extension, the Sangam) grants immense peace to the soul. Do not feel disheartened if the full quantity is not available; perform the rite with what you have.